It would be desirable to ascertain the exact period of an invention of such singular uti lity as this last; one that diffuses its advantages so widely, and that contributes so much to the solace and comfort of old age, by protecting the most intellectual of the senses against the general progress of decay. In the obscurity of a dark age, careless about re cording discoveries of which it knew not the principle or the value, a few faint traces and imperfect indications serve only to point out certain limits within which the thing sought for is contained. Seeking for the origin of a discovery, is like seeking for the source of a river where innumerable streams have claims • to the honour, between which it is impos sible to decide, and where the only thing that can be known with certainty is the boun •ary by which they are all circumscribed. The reader will find the evidence concerning the invention of spectacles very fully discussed in Smith's Optics, from which the most probable conclusion is, that the date goes back to the year 1818, and.cannot with any cer tainty be traced farther.' The lapse of more than two hundred years brings us down to Maurolycus, and to an age when men of science ceased to be so thinly scattered over the wastes of time. Mau tolycus, whose knowledge of the pure mathematics has been already mentioned,- was distinguished for his skill in optics. He was acquainted with the crystalline lens, and conceived that its office' is .to • transmit • to .the optic 'nerve the species of external ob. jests ; and in this process he does not consider the retina ati any way concerned. This theory, though so imperfect, led him nevertheless to form a right judgment of the. de recta of short-sighted and long-sighted eyes. • In one of his first works, Theorenzata Cumin et Umbra, he also gives an accurate solution of a .question proposed by Aristotle, viz. why the light of the sun, admitted through a small hole, and received a, plane at a certain distance from it, always illuminates a round space, whatever he the figure of the hole itself, whereas, through a large aperture, the illuminated space has the figure of the aperture. To conceive the reason of this, suppose than, the figure of the hole is a triangle ; it is plain that at each angle the illuminated space will be terminated by a cir cular arch of which the centre corresponds to the angular point, and the radius to the angle subtended by the sun's semidiameter. Thus the illuminated space is rounded off at the angles ; and when the hole is so small that the size of those ronndings bears a large proportion totthe distance of their centres, the figure comes near to a circle, and may be to appearance quite round. This is the true solution, and the same with that of Mauro .1yeus. The same author appears also to hape observed the caustic curve formed by reflec tion from a concave speculum. .
A considerable step in optical discovery was made at this time by Baptista Porta, 'a Nea-• politan, who invented the Camera Obscura, about the year 1560, and described it in a work, entitled Magia Naturalis. The light was admitted through a small hole in the window-shutter of aglark room, and gave an inverted picture of the objects from which it proceeded, on the opposite wall. A lens was not employed in the first construction of
this: apparatus, but was afterwards used ; and Porta went so far as to consider how the ef fect might be produced without inversion. He appears to have been a man of great in genuity ; and though much of the Magia Naturalis is directed to frivolous objects, it in dicates a great familiarity with experiment and observation. It is remarkable, that we find mention made in it of the reflection of cold by a speculum, an experiment which, of late, has drawn so much attention, and has been supposed to be so entirely new. , The cold by making the focus fall on the eye, which, in the absence of the thermo meter, was perhaps the best measure of small variations of temperature. Porta's book was extremely popular ; and when we find it quickly translated into Italian, French, Spanish, and .Arabic; we see how much the love of science was now excited, and what effects the art of printing was now beginning to produce. Baptista Porta was a man of fortune, and his house was so much the resort of the curious and learned at Naples, that it awakened the jealousy with which the court of Rome watched the progress of improvement. How grievous it is to observe the head of the Christian church in that and the succeeding age, like the Anarch old in Milton, reigning in-the midst of darkness, and complain ing of the _encroachments which the .realm of light was continually making on his ancient ! The constitution of the eye, and the functions of the different parts of which it consists, were not yet fully understood. Maurolyeus had nearly discovered the secret, and it was but a thin, though, to him, an impenetrable veil, which still concealed one important part of the truth. This veil was drawn aside by the Neapolitan philosopher ; but the complete dis covery of the truth was left to Kepler, who, to the glory of finding out the true laws of the planetary system, added that of first analysing the. .whole •scheme of nature in the structure of the eye. He perceived the exact resemblance of this organ to the, dark cham ber, the rays entering the pupil being collected by the crystalline lens, and the other hu .
moors of the eye, into foci, which paint on the retina the inverted images of external objects. By another step of the process, to which our analysis can never be expected to extend, the mind perceives the images thus formed, and refers them at the same time to things without.
It seemed a great difficulty, that, though the images be inverted, the objects are seen erect ; but when it is considered that each point in the object is seen in the direction of the line in which the light passes from it to the retina, through the centre of the eye, it will appear that the upright position of the object is a necessary consequence of this ar rangement.
Kepler's discovery is explained in his Paralipomena in Vitellionemi (Remarks on the Optics of Vitello), a work of great genius, abounding with new and enlarged views, though mixed occasionally with some unsound and visionary speculations. This book appeared in 1604. In the next article we shall have occasion to return to the consideration of other parts of Kepler's optical discoveries.