The reputation ol James Gregory, however, was not colifined to the invention of the telescope. He had alio the high honour of discovering the law of refraction in its most improved form, as we have already stated in our account of his Ily comparing the tables of re fraction in Viteilo's optics, and those given by Atha nasius Kircher, lie was led to the discovery of the con stant ratio of the sines, lly comparing this law with Vitello and Kircher's experiments, lie found the great est deviation to be 1° 29' in water, 1° 50t in wine, I° 44' in oil, and 1° 33' in glass ; but by comparing it with a series of observations ',lack: by himself on the. refractive powers of all these substances, he found the greatest error to be only I 5'. Vitellornade the refractive power of water 1.306, whereas Gregory found it to be 1.3347, within 0 00t8 of the most accurate measure that has yet been published. Gregory learned before the pub lication of his work, that he had been anticipated by Descartes in this discovery, and lie ascribes his igno rance of Descartes's Dioptrics to the " want of new mathematical books in the otherwise celebrated library of Aberdeen " ' Among the optical writers of the present period, was Francis Maria Grimaldi, a learned Italian Jesuit, who, along with his friend Riccioli, cultivated the sciences with much ardour. Ile is principally distinguished for his discovery of the inflexion, or the difframon as he called it, of which lie published an account in his Phisico-mathesis dr Lunatic, Coloribus et Iride, allisyuc annexis, lib ii. which appeared in 1665, at 13ologna, after his death. Having. admitted a ray of light into a dark room by a very small aperture, lie observed that it was diffused in the form of a cone, and that all bodies placed in this divergent light, had their shadows la' ger than they should have been, had the light passed in straight lines by their edges. Upon more mature ob servation, he discovered that the shadow was surround( d uy three coloured fringes, growing narrower as they re ceded from the body ; and, when the light was strong, he perceived similar coloured streaks within the sha dow, which sometimes were only two in number, and somctimes four, the number increasing in the same shadow when it was received at a greater distance from the body. From these new and important facts, Gri maldi concluded that light is bent from its rectilineal direction in passing by the edges of bodies. By ad mitting into a dark chamber two cones of light through apertures placed so near one another, that the one cone did not penetrate the other, till at a considerable dis tance frem the aperture, Grimaldi observed that the rnutual interferences of the rays influence one another, in such a manner as to rcnder the spot illuminated by their joint incidence more obscure than when it was illumi nated by either of them singly. This remarkable result is separately announced in the proposition, "that a body actually illuminated may become more obscure by add ing a new light to that which it already receives." Gri maldi had likewise the merit of first obsert,ing the fact, that the solar image was lengthened by the refraction of the prism ; but he was entirely ignorant of the dif ferent refrangibility of light. See the article GRIMALDI.
Our ingenious countryman, Dr. Hooke, made neirly the same discovery with Grimaldi respecting the in flexion of light, without knowing what had been done by the Italian philosopher. He communicated his ob servations to the Royal Society, in 1672, six years after the appearance of Grimaldi's work ; and he speaks of his paper as containing " the discovery of a new pro perty of light not mentioned by any optical writers be fore him." In another paper, read to the same learned body in 1675, he draws the following conclusions from his experiments, some of which, but particularly the last, have acquired no inconsiderable interest from the investigations of subsequent authors.
1. There is a defection of light, differing both from reflexion and refraction, and seeming to depend on the unequal density of the constituent parts of the ray, whereby the light is dispersed from the place of enn densation, and larefied, or gradually diverged into a quadrant. 2. This deflection is made towards the su perficies of the opaque body perpendienlarly. 3. Those parts of the diverged radiations, which are deflected by the greatest angle from the straight or direct radiation, are the faintest, and those that are deflected by the least angles are the strongest. 4 Rays cutting each other in
one common foramen do not make the angles at the vertex equal. 5. Colours may be made without refrac tion. 6. The diameter of the sun cannot be taken with common sights. 7. The same rays of light, falling upon the same point of an object, will turn into all sorts of colours by the various incLination of the object. 8. Co lours begin to appear when two pulses of light are blended so well, and so near together that the sense takes them for one.
One of the greatest optical and astronomical disco veries of the 17th century, was that of the successive propagation of light, and the velocity with which it moves in absolute space. This interesting truth, which was one of the hest results of the invention of the tele scope, was obtained by the Danish astronomer Olaus Roemer. Galileo, and after hint the Academy del Ci mento, had in vain attempted to measure the speed of this fleet element. While examining some singular ir regularities in the emersion of the first satellite of Ju piter, on the 9th November, 1676, Cassini and Roemer concluded that it depended on the distance of Jupiter from the earth, and that it was necessary for its expla nation, to suppose that the light of the satellite spent about ten or eleven minutes in crossing the orbit ol the earth. Cassini, however, who seems to have first started this hypothetical opinion, speedily abandoned it ; but the notion was keenly maintained by Roemer, who finally succeeded in demonstrating, to the satisfaction of all philosophers, that light moved through the diame ter of the earth's orbit, or about 190 millions of miles in eleven minutes.
Among- the distinguished optical writers of the pre sent petiod was Walter Ernfroy Tschirnhausen, lord of Killingswald and Stolzenberg,, in Lusatia in Saxony. After serving as a volunteer in the Dutch army, in 1672, he travelled through England, France, Italy, and Germany, and having visited Paris for the third time in 1682, he communicated to the Academy of Sciences his discovery ol caustic curves, formed by the refrac tion or reflexion of parallel rays falling upon a curved surface, in consequence of the intersection of the re fracted or reflected rays at points not coincident with the geometrical focus of the refracting or reflecting bo dy. These curves must have been seen by ea•ery person at the bottom of a polished cylindrical vessel, upon the inner surface of which the rays of the sun or of a can dle are incident. In the Memoirs of the Academy for 1682, Tschirnhausen published his first views on the subject, and accompanied them with the delineation of the caustic of a circle formed by incident rays that were Al. De la Hire pointed out some mistakes in this paper, which led to a dispute with its author. This curious subject was afterwards studied by James and John Bernoulli, who extended the theory to converg,ing and diverging rays ; and by M. Bouguer, who discover ed the double caustic, formed at the same time by con vex and concave surfaces. Being the proprietor of ex tensive glass-works, Tschirnhausen availed himself of the opportunity, which was thus afforded him of con stt ucting large burning glasses, of which we have giveD a very particular account in our article on BURNING IN STRUMENTS, VOI. V. I). 140.
Tschirnhausen seems to have been the first person who successfully employed a large object lens as a tele scope, with no other eye-glass but the eye of the ob server. Baptista Porta, Scheiner, and Manfredi Septala of Milan, had no doubt made the same experiment ; but it was left to the Saxon philosopher to carry the idea into actual use. With a double convex object glass 12-1-4,th inches in diameter, and 32 feet long, he saw very distinctly at Nuremberg a whole town about six Eng lish miles distant ; the vision was uncommonly distinct, the light great, and the field of view unusually large. Tschirnhausen had also the merit of establishing in Saxony that porcelain manufacture for which it has so long been celebrated.
About this period a number of very interesting dis coveries relating to vision were made by some ingeni ous French philos.ophers. M. Petit, who devoted much attention to the structure of the eyes of animals, disco vered that the crystalline lens was nearly spherical in serpents and fishes, while in other animals it was lenti cular, the anterior having a greater convexity than the posterior surface. In all animals the crystalline lens grows firmer with age. It has different degrees of hard