These views receive no inconsiderable support from statements contained in Recorde's Pathway to Know ledge, and in the Pantometria and Stratioticos of Leonard Digges. In the first of these works, which was printed in 1551, the author, who is said to have been physi cian io Edward VI. to whom lie dedicates his book, observes, " Great talke there is of a glasse he (viz. Frier Bakon, as he calls him) made at Oxford, in which men might sec things that weare don, and that was judged to be don by power of euill spirites. But I know the reason of it to be good and natural!, and to be wrought by geometry, (with perspective as a part of it,) and to stand as well with reason as to see your face in a common glass." The Pantomctria of Leonard Digges was first printed in London in 1571, and a second edition of it by his son, Thomas Digges, Esq. appeared in 1591. In the preface to this second edition he makes the following curious remarks: " My father, by his continuell pain full practices, assisted by demonstrations mathematical, was able, and sundrie times hath by proportional glasses duely situate in convenient angles, not only discovered things farre off, rcad letters, numbered pieces of mo ney, with the verye coyne and superscription thereof, cast by some of his freends of purpose upon downes in the open fields; but also seven myles off declared what bath been done in private places." In the 21st chapter of the first book, Leonard Digges himself says, " But marvellous are the conclusions that may be performed by glasses (mirrors) concave and convex, of circular and parabolic forms,using for multiplication of beams some times the aid of glasses transposed, which by practice should unite or dissipate the images or figures presented by the reflection of others. By these kind of glasses, or rather frames of them placed in clue angles, yee may not only set out the proportion ef an whole region, yea, represent before your eye the lively image of every house, village, Ste. and that in as little or great space or plan as ye will presente; but also augment or dilate any parcel! thereof, so that whereas at the first appear ance a whole town shall present itself so small and com pact together, that ye shall not discover any difference of streets, yee may, by application of glasses in due pro portion, cause any peculiar house or room thereof dilate ancl show itself in as ample form as the whole town at first appeared, so that yee shall discern any trifle, or read any letter lying there open, especially if the sun's beams may corne into it, as plainly as if you were corporally present, althoug-h it be distant from you as far as eye can descrie. But of these conclusions I mind not here more to intreate, having at large in a volume by itself opened the miraculous effects of perspective glasses." It is impossible, we think, to read the preceding ex tracts, and compare them with the passages from Re corde and Roger Bacon, without being convinced that the camera obscura, and telescopic combinations of mirrors and lenses, had been used in England hefore the time of Baptista Porta and Jansen. It is probable that the mirrors and lenses were placed on stands or in frames, that the objects were seen and magnified by oblique vision, and that on this account it was necessary to place them at convenient angles, as mentioned both by Digges and Roger Bacon. The great respectability and good character, both of Leonard and Thomas Digges, Nvho Nvere grandfather and father to Sir Dud ley Digges, and Nvere descended from an ancient and respectable family, prevent us from supposiug that they could have described inventions which they had not themselves seen.
No farther progress was made in the science of op tics till near the beginning of the sixteenth century, Nvhen Maurolycus of Messina published his Theoremata de Lumine et Umbra, and his Dialzhanorunz Partes, steu Libri Tres. The first of these Nvorks which was com
pleted in 1525, treats of the measure of light, or the illumination of bodies; but it is chiefly remarkable for the solution which INIaurolyeus has given of the pro blem Nvhich had perplexed all optical writers from the lime of Aristotle. It had often been observed, that Nvhen the sun shone through an aperture of any form, the figure of the aperture appeared always round; and in like manner, when the sun had the form of a crescent during an eclipse, the form of every aperture through Nvhich his rays passed was that of a crescent. In ex plaining these phenomena, Maurolycus remarks, that each part of the aperture is the summit of two op posite cones of rays, one of which has the sun for its base, and the other being cut by a plane perpendicular to its axis, %Nil! produce a luminous circle, whose dia meter will be proportional to the distance of the plane of intersection from the aperture. There will, there fore, be painted on the opposite plane as many equal circles of light as there are points in the aperture. Hence if we describe on this plane a figure equal and similarly placed to that of the aperture, and if on each of these points we describe a multitude of circles, the figure Nvhich they will form will be precisely that of the image of the sun. For the vcry same reason the image will be a cresent Nvhen the sun is a crescent.
In explaining the phenomena of vision, Maurolycus considers the crystalline lens as the principal organ, and as transmitting to the optic nerve the images of objects. The Italian philosopher had the merit of dis covering the reason why some persons are long-sighted and others short-sighted. He shows that in the for mer case the rays have not been converged to a focus when they reach the retina ; while in the latter case they have been converged before they reach it. Hence he inds that the convergency may be hastened in long sighted persons, or distinct vision produced, by the use of a convex lens ; and that the convergency may be delayed in short-sighted persons, or distinct vision pro duced, by the use of concave glasses. The fortnation of images at the bottom of the eye seems to have eluded the sagacity of Maurolycus from the difficulty of recon ciling an inverted image with an erect object,—a diffi culty which at a later period perplexed the ingenuity of Kepler himself. Maurolycus attempted, like his pre decessors, to explain the formation of the rainbow; but his labours wcre not crowned with success. He found the sernidiameter of the interior bow 42°, and of the exterior one from to 54° ; but, according to his theo ry, these numbers should have been 45° and 56° ; for he considered the solar ray as partly reflected from the exterior of the drop, and partly entering into the drop, and circulating there by several reflections along the sides of an octagon. He supposed that there were four colours in the rainbow, viz. orange, (crocu.s,)rz,reen, blue, and purple. Maurolycus imagined that he had discover ed the true law of refraction. He supposed that the angle of refraction was proportional to the angle of in cidence, and Nvas always three-eighths of that angle when when the refraction was made from air into glass. Mau rolycus was the first person who observed Nvhat have since been called caustic curves, as formed by refrac tion. He remarked that when light passed through a transparent sphere, a curve was formed by the continu ed intersections of the refracted rays, and that the rays did not converge to one point; and he observed that the rays incident at the greatest distance from the axis, again met the axis after their refraction, at points near er the sphere than those rays which were incident near the axis. By this means a sort of cone of light was ge nerated, having its concave sides formed by the inter section of the different refracted rays.