weighing ten pounds and better is surpassingly more wonderful than that of Moss Seeds of which I have some kinds of them bearing Seeds that a great number of them with their Roots Stalks Leaves and Seeds do not weigh a Grain Besides I have found of the common Female Fern some which have been from the Roots to the utmost top of the Leaf nine foot high and within these three days measured the common broad-leaved Male Fern six foot and an half long some of the Leaves of which are among those I now send you 6. But that which appeared most admirable both to me and some other Gentlemen that were witnesses of it with me was the many differing kinds of small living Creatures wholly invisible to the naked eye and even through largely magnifying spectacles though some of them were to be seen through a deep Convex glass but with a Microscope when the Plant was newly gathered they might be seen nimbly running up and down among the Seed-vessels and some of them were so small as not to be above twice as big as the small Seeds in the bladders a description of some of which I may hereafter send you I have inclosed in the box sent you twelve sorts of Plants of this tribe being the greatest part of the number and only seven sorts of the seeds those wanting are the Cetrach Wall Rue Maiden-hair and Polypody of which notwithstanding you may satisfie your self in the mean time till I can send them green by those small parcels of the Plants which you will find amongst the rest though by keeping they are withered The Seeds of the Ferns through a very excellent Microscope appeared of the higness of a small Vetch or Seed of Lentiles to the naked eye and some of them shrink like the sides of white Pease with small regular knobs and hollows Those of Polypody are differing in colour and shape being yellowish as the others are brown red and formed like the Seeds of the smaller Medicas that is of a Kidney shape All the rest I found very near of the same form I cannot omit what I observed in Cetrach which Plant I have heretofore often considered and wondred at the ill-favoured roughness on the under side of the Leaf appearing like the fleshy side of tann'd Leather being wholly ignorant what Nature meant in it but now by my Microscope I find it a very pleasant object differing from all the rest wherein the curiosity of Nature in a Plant so abject as that appears is shewn beyond imagination This when fresh gathered and not bruised appears through the Microscope like fine thin Membranes such as the Wings of Flies chequered with figures after the manner of Honeycombs when the cells are full of honey and closed with Membranes amongst which as in so many Cells lie the Seed-vessels shaped as before is mentioned I doubt not but you have read the strange stories and fabulous conceits of Authors about Fern Seeds But Parkinson is more Orthodox in some things than any of them For he positively concludes from Gen. 1.11 12. that all Plants have their Seeds and consequently Fern where if he had staid he had asserted a general truth But in coming to particulars he affirms as great an untruth in saying fol. 1036 and 1037. that the Seed is ripe at Midsummer according to the old traditional Fable and tells how it may be gathered whereas now is the very season of their seeding and at Midsummer this and the rest are not come to their full growth before which no Plant seeds That dustiness which he speaks of and calls the Seed is no other than what is found on divers other Plants being an irregular Dust and is not found on the borders of the dents of the Leaves on the under side on which the Seed grows but all over sprinkled on both sides and not found when it is fully grown This he affirms of the Male Ferns which are all differing very notably from the common Female Fern concerning which the fabulous tradition is held But after in the following Chapter of the Ferns and their Relatives now sent you he seems to give over his Scripture Proposition and speaking of the Seeds says no more but that they have spots dashes scales or marks on their back-sides And of the Osmund Royal speaking of the bush at the top of the Plant says it is accounted as the Flower and Seeds And of the Lochitis aspera says plainly they have none at all Of this last I am yet to enquire but doubt not I shall find that it hath Seed like the rest Of all which Gerrard and Johnson his Corrigitor gravely concludes having indeed no demonstrable ground to the contrary that some have been too rash in affirming Ferns to have Seed I intend next Summer to observe whether these hitherto unknown Seeding Plants have Flowers In the mean time I am c. W. C. Bristol September 30. 1669. Maculae in Sole DUring this last great heat of weather in June I observed a very conspicuous Macula with its immediatly incompassing Nubecula and some other less conspicuous Spots at a further distance pass over the Disk of the Sun and found that it was nearest the middle when the heat was greatest that the heat increased as it came nearer the middle and decreased as it departed from it It may be therefore worth observing for the future whether the like weather do not happen upon the next appearance of the like Macula since it seems not very improbable to suppose that the body of the Sun it self may be much hotter when such eruptions appear those Maculae often times ending in Faeculae And the rather because I am informed that this extraordinary heat hath not been peculiar only to England but very general to Europe what it hath been to other parts of the world further intelligence will inform us Upon a second appearance of Spots in the Disk of the Sun at the latter end of July and the beginning of August when at one time to wit July 29. there appeared about six greater and smaller in one knot with their proper Nubecules or Umbra's the heat of the weather again increased to a very great degree and abated as they drew toward the Limb and grew fainter But it hath now since the disappearing viz. on the fourth of August been exceeding hot also though I do not find any Spots this seventh of August it may therefore possibly be that other parts of the body of the Sun may have an extraordinary inflammation which may cause so fervent and lasting heats as have hapned this Summer At least this Hint may deserve some farther Inquiry for though probably it may not be attained to predict the appearances of those Spots yet possibly the appearances of the Spots may serve to predict the future constitution of the weather At least it seems worthy remarking that the greatest heat that hath been in the Air this year was on that day of June when
long it may contain two whole Minutes of such a Circle between f and f and one between 4 and 4 and consequently the said Glass may be set Horizontal to the certainty of a Second which is hardly to be ascertain'd any other way But there remains yet one great Difficulty how to be able to make such a Curviture for though the thing be true in theory yet is it not without some trouble put in practice Very few Glass Canes are so conveniently bent as is desirable and 't is as difficult to find them true straight To prevent this If Glass Canes be used there must be much care taken and many tryals made for the finding what pieces and what side of those pieces will be most fit for this purpose for our Glass-House Workmen know not yet a way certainly to draw them of this or that curviture or straightness nor are they easily ground into a straightness or curviture by the Glassgrinder afterwards though that can be done with some trouble But diligence and tryal will quickly find some piece or other that will be sufficiently exact for any tryal among those which are only drawn at the Glass-House I made use of one of another form such as is described in the 25th Figure which I found to do exceeding well the dark part representing the Water and the lighter part the Air. This was made of two Glasses drawn in distinct Pipes at the Glass-House but joyn'd together in the Lamp and the upper part of the larger or under Tube was incurvated with its convexity downwards so that the Water touched the middle part and the bubbles of Air at each end thereof communicated together by the small Pipe above I tryed also another way by which I was more certain of the truth of the Curvity and could make the Curvity of a greater Circle This was by a long piece of a Looking-Glass-Plate ground very smooth and polished which by the help of Screws I bent upon the circular edges of a brass prismatical Box and cemented the same very tight with hard and soft Cement this Plate had a hollow Channel ground in it the length thereof which serv'd to keep the bubble in the middle By this means 't is not difficult to bend such a Plate into the Curviture of a Circle of 50 60 100 1000 foot Radius and the Brass Box can easily be made to fill or empty as there shall be occasion for the use thereof so that the Bubble may be at any time left of what bigness shall be desired It will be convenient also to varnish the in-side of this Brass Box with Lacker-Varnish very thick and close both to keep it from rustâ—ng and also to preserve it from being corroded by Aqua fortis whensoever there shall be occasion to put it in for the cleansing he inward tarnish and foulness of the Glass-Plate This Curvity of the upper side of the Level may be made by grinding the under side of such a long Plate of Looking Glass upon a Convex Glass-Tool of 50 60 100 1000 foot Radius and polishing the samé accordingly of that Figure The Curvity of the said Plate is express'd in the 26th Figure Now what by this way may be done with Water and Bubbles of Air the same may be done with the same Glasses turned upside-down by the help of an exactly round and polisht Cylinder or Globule of Glass Chrystal Cornelian Agate or other exceedingly hard and close Stone after the manner represented in the 27th Figure for the Ball or Cylinder will naturally roll to the lowest part of the Concavity and there stand But in the doing of this great care must be taken that the Globule be exactly round and polisht and that they Concavity of the Plate be as smooth and well polisht and that they be both very clean and free from dust otherwise the Cylinder or Globule will be apt to stand in a place where it should not and consequently produce considerable errors And here I cannot omit to take notice of a very curious Level invented by Sr. Chr. Wren for the taking the Horizon every way in a Circle Which is done by a large Concave ground and polisht on a very large Sphere and the Limb of it ground and polisht on a flat for by placing the same Horizontal and rectifying it by a small quantity of Quick-silver poured into the Concavity thereof 't will be easie by looking by the flat polisht Limb to discover the true Horizon The only inconvenience I find in it is that the ☿ hath some kind of sticking to the Glass but a small Chrystal Bowl I suppose may remedy that inconvenience and make it fit for use The 5th thing wherein this Instrument is made to excell others is in its easinesses to be adjusted to the Objects and in this that being once adjusted the whole Instrument is so order'd as that it will remain constant to those Objects though they are moved The want of this is so great an inconvenience in all other Instruments hitherto made use of that almost all Observations have been thereby vitiated And Hevelius to prevent and obviate this hath found out many contrivances but they are such as though they do it in part yet 't is but in part and that with much trouble and inconvenience I need not spend time to shew how many inconveniences his way by 4 several Hand-Screws to be managed by 2 Observators at the least is subject to they are indeed so many and so great that it was not without very good reason that he so often appeals to experience for the truth is there was great need of long practice and much experience to be able to make an Observation in that way well the removal of every one of those Screws having an influence upon every one of the other so as no Screw could be turn'd but the whole Instrument was put out of its due situation and both the Objects being continually in motion the whole Instrument was to be rectifi'd every moment There was therefore necessary so great a judgement and dexterity to manage every one of those Screws that without an acquired habitude and handiness by long practice and experience nothing could be done to any certainty nay not even to that little accurateness that the common Sights are able to reach But this though it were a very great unhappiness to Hevelius that he was not furnished with better Contrivances yet it no ways tends to his dispraise for his most extraordinary and indefatigable care pains and industry is so much the more to be admired esteem'd and honour'd and will be so much the more by such as have by experience found the difficulty of making any one Observation certain in that way But that he or any other that hath a mind to make further Tryals and Observations may be freed from this intollerable trouble and difficulty I have thought of this following Instrument by means whereof the Quadrant being once adjusted and
whereby distant places sometimes appear and sometimes disappear under the Horizon By this means also the Rotundity of the Earth may be truely found vastly surpassing any thing performed by the best Levels yet known To this we may add the height of Hils if their distance be known or their distance if their height be known I could have enlarged upon these and have named divers others but designing it only as an Answer to such as may captiously put such a Question I shall rather leave the pleasure of finding them to such as shall really seek them to be assisted thereby in their own undertakings FINIS Errata PAg. 2. l. 13. r. 9 10 p. 6. l. 14. r. aquilae p. 13. l. 3. r. Mathematician p. 15. l. 11. r. Fig. 32. p. 13. l. 28. r. Fig. 31. p. 18. l. 39. r. structuram p. 21. l. 26. r. dena minuta p. 21. l. 27. r. discriminatim p. 22. l. 3. r. Fig. 35. p. 28. l. 34. r. quaedam p. 32. l. 21. r. shaking p. 33. l. 8. r. focus p. 39. l. 28. r. res p. 40. l. 11. r. admaveant p. 40. l. 39. dele se A DESCRIPTION OF HELIOSCOPES And some other INSTRUMENTS MADE BY ROBERT HOOKE Fellow of the Royal Society Hos ego c. Sic vos non vobis LONDON Printed by T. R. for John Martyn Printer to the Royal Society at the Bell in St. Pauls Church-yard 1676. A DESCRIPTION OF HELIOSCOPES And some other INSTRUMENTS THE necessary avocations of business and the urgent importunity of some for the speedy publication of my Animadversions made me conclude them in the Eleventh sheet without staying to Explicate several things which I designed to go along with them But having now retrieved a little more of leasure both for Delineation and Description for a further elucidation of what I have said I shall make it my third Attempt to explain First A Helioscope to look upon the body of the Sun without any offence to the Observers eye Secondly A way of shortning reflective and refractive Telescopes Thirdly A way for using a Glass of any length without moving the Tube Fourthly An Instrument for taking the Diameters of the Sun Moon and Planets or for taking any other Distances to five or ten Degrees to the certainty of a Second Two of these I promised in the 78th or last page of my Animadversions and the other fall in as analogous to them Fifthly An Instrument for describing all manner of Dials by the tangent projection Sixthly The uses thereof 1. For adjusting the Hand of a Clock so as to make it move in the shadow of a Dial whose style is parallel to the Axis Or 2. In the Azimuth of any Celestial Body that is in the shadow of an upright or any other way inclining Style upon any plain 3. For making a Hand move according to the true aequation of Time 4. For making all manner of Elliptical Dials in Mr. Foster ' s way c. 5. For communicating a circular motion in a Curve Line without any shaking And for divers other excellent purposes And first For a HELIOSCOPE which shall so take off the brightness of the Sun as that the weakest eye may look upon it at any time without the least offence My contrivance is By often reflecting the Rayes from the surfaces of black Glasses which are grownd very exactly flat and very well polished so to diminish the Radiations that at length they become as weak and faint as those of the Moon in the twilight so that one may with ease and very much pleasure view examine and describe the phase of the Sun and the maculae and faculae thereof if any such happen to appear when the Observation is made and it gives a good opportunity of discovering them before we have any advertisement thereof from others The reason of which will be sufficiently plain to such as consider how great a quantity of the rays of Light is lost by every reflection and that every reflection doth duplicate triplicate quadruplicate quintuplicate c. the first proportion of loss For Instance Suppose I have a Helioscope made of an Object Glass an Eye Glass and four Reflecting Glasses and that by the first reflection I lose ¾ of the Direct light I affirm there will remain but 1 256 part of the Direct rays of the Sun which can fall upon the eye at the last for if every reflection doth lose ¾ of its Rays and reflect but ¼ and that quarter loseth ¾ and reflects only ¼ of its received Light there will remain but 1 16 part of the whole and if this sixteenth part loseth three quarters of its Rays and reflects only a fourth it will follow the remainder will only be 1 64 part of the whole and if that be once more reflected the Ray will return but with 1 2â—6 part of its first light This although it be obvious and easie enough now it is known yet I do not find that any Person hath yet had thoughts of applying it to this use The generality of Observers have hitherto made use of either some very opacous and thick Glasses next the Eye whether of red green blew or purple Glass others have diminished the Radiation by covering the Glasses with a very thick and close coat of the soot of a Lamp others by casting the figure upon a piece of white Paper whence 't is reflected to the eye Others have contracted the Aperture into a less circle and thereby let in less Light and so make use of one single Ray instead of a pencil of Rayes Others have expanded the figure of the Sun by the help of Eye Glasses into a circle of ten twenty or an hundred times its Diameter But none of all these waies do come near this which I now describe by the help of three four or more Reflections as any one upon trial will very plainly discover First As to the coloured Glasses I cannot at all approve of them because they tinge the Rayes into the same colour and consequently take off the truth of the appearance as to Colour besides it superinduces a haziness and dimness upon the Figure so that it doth not appear sharp and distinct The same inconvenience is also produced by Monsieur Hugenius's way of covering the Glass with the soot of a Lamp though not to so great a degree The Figure on paper or a smooth white surface is not magnified enough nor the difference of shadows so very distinct though that doth very well if the surface be very smooth and the Object be magnified by a Hand Glass That by the contracted Aperture is the worst of all by reason of a certain propriety of Light not taken notice of yet by Optick Writers the edges of Objects seeming ragged of which I have hinted somewhat in my Animadversions pag. 35 and shall shortly say much more the whole ground of Opticks depending thereon The way of expanding the figure of the Sun by the Eye Glass
to me seems the best of all the rest but that is apt to vitiate the Figure to super-induce somewhat of Colour and doth not give the smallest distinctions of lights and shadows without somewhat of colour and somewhat of haziness and dimness The Glasses of this HELIOSCOPE may be made either by refracting or reflecting Spherical Glasses The best way for taking in a large Angle is the using refracting Glasses both for the Object and Eye Glasses but the best way for taking in a small part and for avoiding haziness dimness and colours is by Reflection either in part or in whole that is either to make the Object Glass only by way of Reflection and the Eye Glass by that of Refraction or both the Object-glass and Eye Glass also by reflection and to have no refraction at all The several waies of doing which I have represented in the adjoyning Table wherein I have expressed ten several waies of placing the several Glasses so as to be fit for the use designed The first way represented in the first Figure is a sixty foot Object-Glass contracted into a twelve foot Tube by the help of four several Reflecting-plates placed between the Object-Glass and Eye-Glass The Experiment of doing which I produced and shewed before the Royal Society at divers of their publick Meetings at Arundel house in the year 1668 and it remains upon their Register This as I then shewed would be of exceeding great use in all manner of Perspectives and Telescopes if we could find a good material that would make the Reflections very strong and full And that would not be subject to lose its Figure which all our specular Mettals are very apt to do for by it 't would be possible to contract the Tubes for long Glasses into very short lengths and so make them of easie use and manage This I attempted with several sorts of Mettal made with ♃ ♀ ♂ Antimony and Arsenick but most of these compound Mettals I found to be very spongy and consequently in the last polish to receive though a very glaring polish yet such as did much confound the Object by a kind of haziness especially if Putty be used to glase it and for this purpose Putty must not in any wise that I yet know of be used it being so very apt to round off the edges of pores or scratches which does much contribute to the haziness and confusion of the Object If I made use of Glasses foil'd with Quicksilver which I found to give much the best reflection yet I found this inconvenience that a considerable part of the Ray was lost by the double reflection at the unfoil'd superficies of the Glass The first from the surface of the Glass before it entred this as it weakned the Ray so mingling with the other reflection that came from the bottom it created some kind of haziness and confusion if the two superficies of the Glass were parallel but if they were not parallel it superinduced somewhat of Colour unless it were helped by a contrary refraction in a second Reflecting-glass after the manner of that which is delineated in the fifth Figure where let a b represent the Object-Glass c g the first Reflecting-plate whose thinnest side is to c and d θ the second Reflecting-plate whose thinnest part is towards θ which doth thereby take off the first Refraction of c g and destroy the Colours superinduced by the first The Ray also was weakned much more from the second reflection it suffered at the unfoil'd superficies of the Glass from the reflection of the Air or aether which is much stronger than that of Glass at its re-entring into the Air. Besides this I find that the substance of most Glass is so imperfectly mixt that there is in the very best much of veinyness and inequality of Refraction in the parts thereof and thence though there were no visible vein appearing in the body of the Glass and though both the surfaces thereof were very truly figured and polished yet there was some kind of dimness superinduced upon the Objects by the rays passing through those Glasses But this was not in all for I found some that did very well answer my expectation and I am very apt to believe that if a pot of Glass were made on purpose by a way I know the body thereof might be made perfectly clear uniform and transparent without blebs veins or sands which when I have leasure and opportunity I design to experience farther But this only by the by in relation to the shortning the Tubes of Telescopes for the Moon Planets and other Objects because it is not at all to our present purpose of making a Helioscope where we make use only of the reflection of the first superficies of the Glass and where our main aim and design is the loss of the strength and brightness of the Rays and not for preserving the strength and briskness of the Rays or augmenting them And therefore for this use the best material I have yet met with is black Glass black Marble and Glass of Antimony For these substances being very dark and opaque do reflect but a very small part of the Raies that fall upon it and none of those that penetrate into it especially if they be thick and being of a very hard and permanent substance are capable of receiving a very curious and exact polish and qualified sufficiently to retain and keep it without receiving injury from the Air or ordinary wiping But in the making of these Glasses for Long Telescopes very great care and diligence must be used to make them of a true flat and so much the more by how much the nearer they are placed to the Object-Glass and the further from the Eye-Glass a little err our at a great distance from the eye being vastly magnified to the eye at that distance whereas a greater becomes insensible if it be near the eye Let a b in the first represent a sixty foot Glass whose focus is at o let a c d e f o and b g h i k o represent the two side Rayes of the pencil of light this Pencil by the four Reflecting surfaces γ η δ θ ε ι ζ κ is broken into five shorter lengths η δ answering to c d γ θ to g h δ ε to d e θ ι to h i ε ζ to e f and ι κ to i k and lastly ζ ο and κ ο to f o and k o as will be sufficiently plain to any one that will but consider the Scheme By this way four fifths of the length of the Tube is taken away which is the most that can be taken away by four Reflections every reflection running the whole length of the Tube a lesser part of the length may be taken away in any proportion assigned as in the second contrivance described in the second Figure two thirds are taken off when the same Letters answer to the Object-Glass Eye-Glass the flexures of the side Rays of the Pencil and the