Part 7
Together with my most hearty thanks for the favour you were pleased to do me, in sending me an _Epitome_ of what had been by the ingenious Monsieur _Auzout_ animadverted on a description, I had made of an _Engine_ for _grinding spherical Glasses_, I thought my self obliged, both for your satisfaction, and my own Vindication, to return you my present thoughts upon those Objections. The chief of which seems to be against the very _Proposition_ it self: For it appears, that the _Objector_ is somewhat unsatisfied, that I should propound a thing in _Theory_, without having first tried the _Practicableness_ of it. But first, I could wish that this worthy Person had rectified my mistakes, not by speculation, but by experiments. Next, I have this to answer, that (though I did not tell the _Reader_ so much, to the end that he might have the more freedom to examine and judg of the contrivance, yet) it was not meer _Theory_ I propounded, but somewhat of _History_ and _matter of Fact_: For, I had made trials, as many as my leisure would permit, not without some good success; but not having time and opportunity enough to prosecute them, I thought it would not be unacceptable to such, as enjoyed both, to have a description of a way altogether _New_, and _Geometrically_ true, and seemingly, not unpracticable, whereof they might make use, or not, as they should see reason. But nothing surprised me so much, as, that he is pleased (after he had declared it a fault, to write this _Theory_, without having reduced it to practice) to lay it, as he seems to do, in one place of his book, _p._ 22 upon the _Royal Society_. Truly, _Sir_, I should think my self most injurious to that _Noble Company_, had I not endeavoured, even in the beginning of my Book, to prevent such a misconstruction. And therefore I cannot but make this interpretation of what Monsieur _Auzout_ saith in this particular, that either he had not so {65} much of the Language wherein I have written, as to understand all what was said by me, or, that he had not read my _Dedication_ to the _Royal Society_, which if he had done, he would have found, how careful I was, that that _Illustrious Society_ should not be prejudiced by my _Errors_, that could be so little advantaged by my _Actions_. And indeed, for any man to look upon the matters published by their Order or Licence, as if they were _Their_ Sense, and had _Their_ Approbation, as _certain_ and _true_, 'tis extremely wide of their intentions, seeing they, in giving way to, or encouraging such publications, aim chiefly at this, that _ingenious conceptions_, and important _philosophical matter of Fact_ may be communicated to the learned and enquiring World, thereby to excite the minds of men to the examination and improvement thereof. But, to return; As to his _Objections_ against the _Matter_, I do find that they are no more against mine, than any other way of _Grinding Glasses_; nor is it more than I have taken notice of my self in this Passage of the same _Paragraph_, of which sort are also those difficulties he raises about _Long Glasses_, which are commonly known to such, as are conversant in making them _It would be convenient also_ (these are my words) _and not very chargeable, to have four or five several Tools: One,_ &c. _And, if curiosity shall ever proceed so farr, one for all lengths, between 1000. and 10000. foot long; for indeed, the _Principle_ is such, that supposing _the Mandrils well_ made, and of a good length, and supposing _great care_ be used in working and polishing them, I see no reason, but that a Glass of 1000. nay, 10000. foot long may be made, as well as one of 10. For, the reason is the same, supposing the _Mandrils_ and _Tools_ be made sufficiently strong, so that they cannot bend; and supposing also that the Glass out of which they are wrought, be capable of so great a regularity in its parts, as to its Refraction._ But next, I must say that his _Objections_ to me, seem not so considerable, as perhaps he imagines them. For, as to the possibility of getting Plates of Glass thick and broad enough without veins, I think _that_ not now so difficult here in _England_, where I believe is made as good, if not much better Glass for _Optical Experiments_, than ever I saw come from _Venice_. Next, though it were better, that the thickest part of a long _Object-Glass_ were exactly in the middle, yet I can assure Monsieur _Auzout_, that it may be a very {66} good one, when it is an Inch or two out of it. And I have a good one by me at present, of 36. foot, that will bare an _Aperture_, if _Saturn_ or the _Moon_ in the _twilight_, be look'd on with it, of 3½ Inches over, and yet the thickest part of the Glass is a great way out of the middle. And I must take the liberty to doubt, whether ever my _Animadversor_ saw a long Glass, that was otherwise; as he might presently satisfie himself by a way I could shew him (if he did not know it) whereby the difference of the thickness of the sides might be found to the hundreth part of a Line.
As to the exceeding exactness of the _Figure_ of Long _Object-Glasses_, 'tis not doubted, but that it is a matter difficult enough to be attained any way: but yet, I think, much easier by _Engine_, than by _Hand_; and of all _Engines_, I conceive, none more plain and simple, than that of a _Mandril_. And for making _spherical Glasses_ by an _Engine_, I am apt to think, there hardly can be any way more plain, and more exact, than that which I have described; wherein there is no other motion, than that of two such _Mandrils_, which may be made of sufficient strength, length, and exactness, to perform abundantly much more, than I can believe possible to be done otherwise than by chance, by a man's hands or strength unassisted by an _Engine_, the motion and strength being much more certain and regular. I know very well, that in making a 60. foot Glass by the strength of the hand, in the common way, not one of ten that are wrought, will happen to be good, as I have been assured by Mr. _Reeves_; who, I am apt to think, was the first that made any good of that length. For the _Figure_ of the _Tool_ in that way is presently vitiated by the working of the Glass, and without much _gaging_ will not do any thing considerable. Besides, the strength of a man's hands, applied to it for the working and polishing of it, is very unequal, and the motions made, are very irregular; but in the way, I have ventured to propose, by _Mandrils_, the longer the _Glass_ and _Tool_ are wrought together, the more exact they seem to be and if all things be ordered, as they should be, the very polishing of the Glass, does seem most of all rectifie the _Figure_.
As to what he objects, that the Tool does only touch the Glass in a _Mathematical Circle_; that is true, perhaps, at first, but before the Glass is wrought down to its true _Figure_, the _Edge_ of the _Tool_ {67} will be worn or grownd away, so as that a Ring of an inch broad may be made to touch the _Spherical Surface_ of the Glass; nay, if it be necessary (without much trouble, especially in the grinding of longer Glasses) the whole _Concave Surface_ of the _Tool_ may be made to touch a Glass. Besides, that as to the keeping a quantity of the same sand and Powders of several finesses, according as the glass wears, the same is possible to be don, as with the same Sand wrought finer by working in the Ordinary way.
The giving the _Inclination_ to the _Mandrils_, is not at all difficult; though perhaps to determine the length exactly which the Glass so made shall draw, is not so easie: But 'tis no matter, what length the Glass be off, so it be made good, whether 60 or 80 foot, or the like. Nor is it so very difficult, to lay them both in the same _Plain_. And to keep them _steddy_, when once fix'd, is most easie.
As to the Calculation of the propriety of a Glass of a thousand foot, perhaps for that particular Length, I had not, nor have as yet calculated, that the Convexity of one of eighteen inches broad, will not be above a seventh part of a Line. But it does not thence follow, that I had not considered the difficulties, that would be in making of it. For, I must tell him, that I can make a _Plano convex_ Glass though its convexity be a smaler sphere than is usual for such a length to be an _Object-Glass_ of about 150 foot in Length, nay of 300 foot, and either longer or shorter, _without_ at all _altering the convexity_. So that, if he will by any Contrivance he hath, give me a _Plano-convex_ Glass of 20, or 40 foot _Diameter_, without _Veins_, and truly wrought of that _Figure_, I will presently make a _Telescope_ with it, that with a single Ey-glass shall draw a thousand foot: Which _Invention_, I shall shortly discover, there being, I think, nothing more easie and certain. And if a _Plano-convex_ Glass can be made of any _Sphere_ between twenty and fourty foot _radius_, so as that both the _Convex_ and _Plain_ side of the Glass be exactly polish'd of a true _Figure_, I will shortly shew, how therewith may be made a _Telescope_ of any Length, supposing the Glass free from all kind of _Veins_, or inequality of _Refraction_.
As for the sliding of the Glass upon the _Cement_, I see no reason at all for it, at least in the _Cement_, I make use of, having never observed any such accident in hard _Cement_. {68}
And for the Bearing of the _Ring_ against one side of the Glass only at a time, I cannot see, why _that_ should produce any inequality, since all the sides of the Glass have successively the same pressure.
His ratiocination concerning a Glass of 300 foot, is much the same with the former, about the difficulty of working a true surface of a convenient figure; which how considerable both _that_ and his Conclusion thereupon (_videl. That we are not to expect Glasses of above 300 or 400 foot long at most, and that neither _Matter_ nor _Art_ will go so far_) is, may be judged from what I have newly told you of making any _Object-Glass_ of any Length.
And for his good wishes, that those, who promise to make him see _Plants_ or _Animals_ in the _Moon_ (of which I know not any, that has done so, though perhaps there may be some, notwithstanding his Objections, that do not yet think it impossible to be done) had considered, what a Man is able to see with his _bare_ Eye at 60 Leagues distance: I cannot but return him my wishes, that he would consider the difference between seeing a thing through the _Gross_ and _Vaporous_ Air neer the Earth, and through the Air over our heads: Which, if he observe the Moon in the _Horizon_, and neer the _Zenith_ with a _Telescope_, he will experimentally find; and, having done so, he will perhaps not be so dissident in this matter.
Concerning his Advertisement to such, as publish _Theories_, I find not, that he hath made use of it in his own case. For, in his _Theory_ about _Apertures_ he seems to be very positive, not at all doubting to rely upon it, _vid._ that the _Apertures_ must be _thus_ and _thus_ in _great_ Glasses, because he had found them _so_ or _so_ in some _small_ ones.
For his Proposal of amendments of some inconveniencies in this way, I return him my thanks; but as to his first I believe, that the matter may be conteined as wel in the _Concave_ Tool, as on the _convex_ Glass. And as to that of 2 _Poppet-heads_ I do not well understand it, if differing from mine; and the keeping of the Tool upon the Glass with a spring or weight, must quickly spoyl the whole; since, if either of the _Mandrils_ will easily yield backwards, the _regularity_ of _all_ will be spoiled: and as to the wrigling and playing of the _Mandril_, I do not at all apprehend it. {69}
His _Theory_ of _Apertures_, though he seems to think it very authentick, yet to me it seems not so cleer. For, the same Glass will endure greater or lesser _Apertures_, according to the lesser or greater Light of the _Object_: If it be for the looking on the _Sun_ or _Venus_, or for seeing the _Diameters_ of the _Fix'd Stars_, then smaller _Apertures_ do better; if for the _Moon_ in the _daylight_, or on _Saturn_, or _Jupiter_, or _Mars_, then the largest. Thus I have often made use of a 12 foot-Glass to look on _Saturn_ with an _Aperture_ of almost 3 inches, and with a single Eye-glass of 2 inches _double convex_: but, when with the same Glass I looked on the _Sun_ or _Venus_, I used both a smaller _Aperture_, and shallower _Charge_. And though M. _Auzout_ seems to find fault with the _English_ Glass of 36 foot, that had an _Aperture_ of but 2¾ inches _French_; as also, with a 60 foot _Tube_, used but with an _Aperture_ of 3 inches; yet I do not find, that he hath seen Glasses of that length, that would bear greater _Apertures_, and 'tis not impossible, but his _Theory_ of _Apertures_ may fail in longer Glasses.
* * * * *
_Of a means to illuminate an Object in what proportion one pleaseth; and of the Distances requisite to burn Bodies by the _Sun_._
One of the means used by M. _Auzout_ to enlighten an Object, in what proportion one pleaseth, is by some great _Object-Glass_, by him called a _Planetary_ one, because that by it he shews the difference of Light, which all the _Planets_ receive from the _Sun_, by making use of several _Apertures_, proportionate to their distance from the _Sun_, provided that for every 9 foot draught, or thereabout, one inch of _Aperture_ be given for the _Earth_. Doing this, one sees (_saith he_) that the Light which _Mercury_ receives, is far enough from being able to burn Bodies, and yet that the same Light is great enough in _Saturn_ to see cleer there, seeing that (to him) it appears greater in _Saturn_, than it doth upon our _Earth_, when it is overcast with Clouds: Which (he adds) would scarce be believed, if by means of this Glass it did not sensibly appear so; Whereof he promises to discourse more fully in his {70} _Treatise of the usefulness of great Optick-Glasses_, where he also intends to deliver several Experiments, by him made, 1. Touching the quantity of Light, which a Body, that is 10, 15 and 20 times, &c. remoter than _Saturn_, would yet receive from the _Sun_. 2. Touching the quantity of Light, by which the _Earth_ is illuminated even in the _Eclipses_ of the _Sun_, in proportion of their bigness. 3. Touching the quantity of Light, which is necessary to burn Bodies: he having found, that not abating the Light, which is reflected by the Surfaces of the Glass (whereof he confesseth, he doth not yet exactly know the quantity) there would be necessary about 50 times as much Light, as we have here, for the burning of _Black_ Bodies; and neer 9 times more for the burning of _White_ Bodies, than for the burning of _Black_ ones: and so observing the immediate proportions between these two, for burning bodies of _other_ Colors. Whence (he tells us) he hath drawn some consequences, touching the distance, at which we may hope, to burn Bodies here, by the means of _great Glasses_ and great _Looking-glasses_. So that (_saith he_) we must yet be seven times neerer the _Sun_, than we are, to be in danger of being burned by it. Where he mentions, that having given _Instructions_ to certain persons, gon to travel in _Hot Countries_, he hath among other particulars recommended to them, to try by means of great _Burning-glasses_, with how much less _Aperture_ they will burn _there_, than _here_, to know from thence, whether there by more Light _there_ than _here_, and how much; since this perhaps may be the only means of trying it, supposing, the same matters be used: although the difference of the Air already heated, both in _hot Countries_, and in the _Planets_, that are neerer than we, may alter, if not the quantity of Light, at least that of the Heat, found there.
* * * * *
_A further Account, touching Signor _Campani_'s Book and Performances about _Optick-glasses_._
In the above-mentioned _French_ Tract there is also conteined M. _Auzout's_ Opinion of what he had found New in the _Treatise_ of Signor _Campani_, which was spoken of in the first _Papers_ of these _Transactions_, concerning both the Effect of the _Telescopes_, contrived after a peculiar way by the said _Campani_ at _Rome_, and {71} his New Observations of _Saturn_ and _Jupiter_, made by means thereof.
First therefore, after that M _Auzout_ had raised some scruple against the Contrivance of Signor _Campani_ for making _Great Optick-Glasses_ without _Moulds_, by the means of a _Turn-lath_, he examines the _Observations_, made with such _Glasses_: Where, having commended _Campani_'s sincerity in relating what he thought to have seen in _Saturn_, without accomodating it to M. _Hugens_'s _Hypothesis_, he affirms, that supposing, there be a _Ring_ about _Saturn_, Signor _Campani_ could not see in all those different times, that he observed it, _the same Appearances_, which he notes to have _actually_ seen. For, having seen it sometimes in _Trine Aspect_ with the _Sun_, and _Oriental_; sometimes, in the same _Aspect_, but _Occidental_; sometimes in _Sextil Aspect_, and _Occidental_; at another time, again in _Trine_, and _Oriental_, this Author cannot conceive, how _Saturn_ could in all these different times have no difference in its _Phasis_, or keep always the same _Shadow_; seeing that, according to the _Hypothesis_ of the _Ring_, when it was _Oriental_, it must cast the _Shadow_ upon the _left_ side of the _Ring_ beneath, without casting any on the _right_ side: and when it was _Occidental_, it could not but cast it on the _right_ side beneath, and nothing of it on the other.
Concerning the _Shadow above_, which _Campani_ affirms to be made by the _Ring_ upon the Body of _Saturn_, M. _Auzout_ judges, that there could be no such _Phænomenon_, by reason of its _Northern Latitude_ at the times, wherein the _Observations_ were made, _vid._ in _April_ 1663; in the midst of _August_, and the beginning of _October_, next following, and in _April_ 1664, except it were in _October_, and the _Shadow_ strong enough to become _visible_.
But as to the _Shadow below_, he agrees with _Campani_, that it does appear, yet not as he notes it, seeing that it must be sometimes on the one side, sometimes on the other; and towards the _Quadrat_ with the _Sun_ it must appear biggest, as _indeed_ he affirms to have seen it himself _this_ year, insomuch that sometimes it seemed to him, that it covered the whole _Ring_, and that the _Shadow_, joyning with the obscure space between both, did interrupt the circumference of the _Ring_; but beholding it at other times in a cleer Sky, and when there was no Trepidation of the Air, {72} he thought, that he saw also the Light continued from without, although very slender. But he acknowledges, that he could never yet _precisely_ determine, by how much the largeness of the _Ring_ was bigger than the _Diameter_ of _Saturn's_ Body. As for the proportion of the Length to the Breadth, he affirms, to have alwaies estimated it to be two and a half, or very neer so; and to have found in his _Observations_, that in _January_ last, one time, the length of _Saturn_ was 12 _Lines_, and the breadth 5. Another time, the length was 12. _Lines_, and the breadth 4. and this by a peculiar method of his own. But yet he acknowleges also, that sometimes he hath estimated it as 7. to 3. and at other times as 13. to 5. and that if there do not happen a change in the magnitude of the _Ring_ (as it is not likely there does) that must needs proceed from the Constitution of the air, or of the Glass's having more or less _Aperture_, or from the difficulty of making an exact estimate of their proportions. However it is not much wide (saith he) of two and a half, although _Campani_ make the length of the _Ring_ but double to its breadth.
Monsieur _Auzout_ believes, that he was one of the first that have well observed this shadow of _Saturn's_ Body upon its _Rings_ which he affirms happened two years since; when, observing in _July_, for the first time, with a _Telescope_ of 21. and then another of 27. foot, he perceived, that the _Angle_ of the obscure space on the _right side_ beneath, was bigger and wider, than the three other _Angles_, and that some interruption appear'd _there_, between the _Ring_, and the _Body of Saturn_; of which he saith to have given notice from that time to all his friends, and in particular, as soon as conveniently he could, to Monsieur _Hugens_.
He confesseth, that he hath not had the opportunity of observing _Saturn_ in his _Oriental Quadrat_; yet he doubts not, but that the _shadow_ appears on the _Left-side_, considering, that the _Existence_ of the _Ring_ can be no longer doubted of, after so many _Observations_ of the _shadow_ cast by _Saturn's_ Body upon it, according as it must happen, following that _Hypothesis_; there being no reason, why it should cast the said _shadow_ on one side, and not on the other.