Parabolic mirrors of a large size, and very considerable power, were constructed by M. Hocsen of Dresden, and afterwards by M. Ehrard. These mirrors were compos ed of several pieces of solid wood, and on the convex part were pieces of wood, both diverging from the ver tex and transversely, nicely fitted and strengthened. The concave part of this framing was covered with copper plate of an inch in thickness, 44 feet long, and 24 feet broad, so as to resemble one piece, finely polished. The speculum was so supported as to be easily managed, and the anterior part of it was subtended by an iron arch half an inch thick. The middle of this arch, which coincid ed with the place of the burning focus, was perforated into a ring, which supported from both sides an iron fork for receiving the body to be examined. Four of Mr Ehrard's mirrors constructed in this way had the fol lowing dimensions : The celebrated Wolfius, who had witnessed the ef fects of these mirrors, assures us, that in burning, calcin ing, melting, and vitrifying, they far exceeded any thing of the kind ever known. The hardest stones scarcely resisted a few seconds. Metals were rapidly perforated, and vegetables and bones were immediately burnt to a cinder and Nitrified.
Our celebrated countryman, Dr James Gregory, turn ed his attention to the construction of burning machines, about the year 1670 ; and in a letter to Mr Collins, dated St Andrews, 7th March, 1673, he states his views on this subject, and requests Mr Collins to communicate them to Sir Isaac Newton, who returns a favourable opinion of the invention, in a letter to Mr Collins. The passages in these letters are too interesting to be given in any other form than in the original words of these dis tinguished authors.
" r Newton's discourse of reflection," says Dr Gre gory, " puts me in mind of a notion I had of burning glasses several years ago ; which appears to me more usefull than subtile. If ther be a concave specuitnn of glasse, the leaded convex surface having the same cen ter with the concave, or to speak preciselie, albeit per chance to little more purpose, let the radius of the con vexitie be c, the thicknes of the glasse in axis transitu + the radius of the convexitie equal to 9c + 5f this speculum sal have the foci of both the surfaces in the same point ; and not onlic that, but all the rays which are reflected betwixt the two surfaces, sal, in their egresse, come, gum nroxime, to the common focus. The making of such an speculum, rcquireth not much more airt than an ordinal• plane glassc, seing great sub tiltic is not necessar here : so that I believe they who mak the plane miroir glasses, wold mak one of these, three foot in diameter, for four or five pounds sterling, or little more: for I have seen plane glasses, almost of that bignes, sold even here for less money. Now sting (as Air Newton observeth) that al reflecting metalls lose more than one-third of the raycs ; nis concave glasse, even cxteris naribus, wold have an great advantage of a metall one ; for certainlie an exactlie polished thin mi roir-glasse, of good transparent miter, after a few reflec tions, doeth not lose one-fourth of the raycs : and, upon other accounts, this bath incomparable advantages, seing it is more portable, free from tarnishing, and, above al, hardlic of the value. The great usefulness of burn ing concaves, this being so obvious, and as yet (for quhat I know) untouched by aide, makes inc jealous that there may be in the practice sonic fallacie. Ye may comma Mcate this to intelligent persons, and especiallie to Mr Newton ; assuring him that none bath a greater venera tion for him, admiring more his great and subtile inven tions, than his and yours.
P. S. If ye please, let me hear, with the first conveni ence, what may he judged the result of this burning con cave ; for I am as much concerned to be undeceived, if titer be any insuperable dillicultie, as to be informed of an most surprising success. I have spoke of it to severals here, but al wer as ignorant of it as my self," &c.
Sir Isaac Newton's reply to Mr Collins is dated Cam bridge, April 9th 1673, and contains the following pas sage :— " The design of the burning speculum appears to me very plausible, and worthy of being put in practice. What
artists may think of it, I know not ; but the greatest difficulty in the practice, that occurs to me, is to propor tion the two surfaces so, that the force of both may be in the same point according to the theory. But, perhaps, it is not necessary to be so curious ; for, it seems to me, that the effect would scarce be sensibly less, if both sides should be ground to the concave and gage of the same tool, &c. &c." The attention of Sir Isaac Newton being thus acciden tally directed to the subject of burning instruments, he procured seven concave glass mirrors, each of which was eleven and a half inches in diameter, and six of these were placed round the seventh, and contiguous, but so as to have one common focus. The general focal length was 22 inches and a half, and about an inch in diameter. It melted gold in about half a minute, and vitrified brick or tile in one second.—The effect of these specula was obviously much less than seven times the effect of any one of them. The rays of the sun could fall perpendi cularly only on the one in the middle ; and, in conse quence of this obliquity of incidence, none of the spe cula intercepted a column of rays of the same diameter, and the image formed in the focus of each could not be exactly circular.* Burning mirrors, composed of glass, were construct ed by M. Zciher of St Petersburgh. His object was to convert plates of plain glass into concave mirrors. which he effected, by placing the glass upon a convex tool, and exposing it to a strong heat, till it assumed the exact curvature of the tool. Zciher made numerous trials with plates of various sizes, and, after several failures, he succeeded in finding the proper method of conducting the operation. No particular difficulties occurred in giving the proper shape to plates five or six inches in diameter ; but, in forming one of 16 inches. the circum ference was moulded to the tool before the central parts, where a number of vesicles of air had collected ; and, in some other cases, the glasses cracked after they had re ceived the proper shape. The following method is that which Zciher always found to succeed : A small bit of the glass to be used, must first be ex posed to the fire till it becomes red hot, and if, after cooling, it has preserved its polish and transparency, the glass is fit for the required purpose, for it sometimes happens that the glass becomes quite black after the operation. The plate of glass is next placed on a con cave iron dish of the required curvature, and put into a furnace. Coals are placed below and above the dish, and on all sides of it. The greatest care must then be taken that the glass shall become equally hot both at the circumference and at the centre, for if the red colour should get deeper in the middle, the glass will be in great danger. As soon as the whole is red hot, the in stant of its bending to the shape of the mould must be carefully watched, and when this happens, which may be observed from the reflected images of the surrounding coals, all the fire must be removed from above the glass, and also a great part of the fire at its sides. The glass must then be covered with warm ashes, that have been passed through a sieve, and it must be allowed to cool gradually. It is of the utmost importance to mark the precise moment when the glass applies itself to the sur face of the mould, for, if it remain too long, a part of the scoria, which separates from the mould, will adhere to the glass. When the glass is covered with the hot ashes, the fire must still be allowed to remain below the mould, lest the glass should crack by being cooled too suddenly. When the glass is taken from the furnace, its convex sides may then be silvered for a burning speculum ; or, if a lens is required, two of the pieces of glass may be joined, so as to contain a fluid.