Contemporary with Alaurolyeus was the celebrated Neapolitan philosopher, Joannes Baptista Porta, who has rendered bimself more famous by bis ardour as a col lector of mysteries, than others have done by the dis covery of new facts. Having instituted what he called an ..,Icademy of Secrets, which met at his own house in Naples, and embraced all the ingenious individuals which that city contained, he spared neither labour nor ex pense in collecting all the remarkable facts and experi ments in physical science. By his own exertions, and those of the members of his academy, he was enabled to compose his Magia Naturalis, a work which, as Mon tucla has observed, is filled with pretended observations, compiled in general with more credulity than judgment. His work appeared in the year 1560, when he was only about fifteen years of age, and a second edition appeared about thirty years afterwards, with numerous alterations and additions. At its first appearance it was translated into French, Italian, Spanish, and Arabic, ancl underwent various editions in different countries ; and thoug-h it is a wonderful collection of every thing temarkable that was known at the time, yet it is principally celebrated in the history of optics for the description which it con tains of the camera obscura. Baptista Porto observes, in the seventeenth chapter, that if a small hole be made in the shutter of a window, the images of all external objects will be represented on the wall in their propel colours ; and that if a convex glass be fixed in the aper ture, so that the images are viewed at the distance of its focal length, they will be rendered much more dis • tinct, so that even the features of a person out of the room may be distinguished in his image on the wall. In order to make the images formed on the camera obscura. erect, he recommends that they should be viewed upon a convex mirror, properly adapted to the convex glass, and kept at a proper distance from the aperture. Not satisfied with the mere exhibition of images of external objects on the wall, Baptista Porta applied his invention to the representation of eclipses of the sun, and of pic tures of drawings which were placed on the outside of the convex lens, and at such a distance that their images were distinctly formed on the wall. By giving motion to these pictures and drawings, he exhibited thc appearance of a hunt, of battles, and of other scenes, which in these times excited no small degree of aston ishment. In studying the images formed in his camera obscura, Baptista Porta could not fail to observe that they were magnified, and, therefore, seen more distinct ly than with the naked eye ; and he accordingly found, by placing his cye behind a convex lens, that he could read a letter which Ile could not read with his naked eye. In this case his eye was placed six or seven in ches behind the image formed by the convex lens, and consequently this instrument was a telescope, in which his eye petformed the part of an eye-glass, as in the instrument which Roger Bacon seems to have employed.
The phenomena ol the camera conducted its inven tor to some just views regarding vision. He maintained that vision is performed by the entrance of rays into the eye ; that the pupil. which he observed to contract and dilate when exposed to different lights, was analogous to the hole in his window-shutter, and that the crystal line lens was the principal organ of It appears, however, that Ile did not regard thc crystalline lens as performing the part of his convex lens, but rather con sidered it as corresponding to the ground which receiv ed the image. This strange mistake is not easily ac counted for. The discovery of the contraction and di latation of the pupil seems to have been made so early as 1522, by Achillinus, and also at a later period by Father Paul of Venice.
In the year 1593, Baptista Porta published another work, which he entitled De Refractione Optices parte libri novenz, and in which he discusses a great variety of optical subjects, sometimes with acuteness and dis ( crnment, and at other timcs in a manner very vague and incorrect. The zeal and ardour of Baptista Porta, are always more prominent than his genius ; and it is rather singular, that a philosopher who knew so much of what was done by others, and who made so many ex periments, should have almost no other claim upon the gratitude of posterity, than the invention of the camera, which, after all, was probably communicated to him.
The subject of the rainbow began about this time to occupy the particular attention of philosophers. J. Clichtoveus, who died in 1543, considered the exterior bow as a reflected image of the interior one, and in this manner endeavoured to account for the faintness of its light, as well as the inverted order of its colours A more important view of the subject was afterwards given by J. Fleschier of Breslau, in a work entitled
De Iridibus doctrina Aristotelis ct Vitellionis, which ap peared in 1571. This author ingeniously ascribes the phenomena of the rainbow to two refractions, and an intermediate reflexion within the drop ; but after the ray had thus acquired its colours, he supposed that it was again reflected from another drop before it reached the eye of thc spcctator. A still nearer approach to the true cause of the phenomenon, was madc by the cele brated Kepler, in a letter to our eminent countryman Harriot, writtcn in 1606. He conceives that a solar ray, which is a tangent to the drop of rain, is first re fracted into the drop, and after snffering a reflexion from the bottom of the drop, is again refracted to the eye of the spcctator. This is no doubt very nearly thc pro gress of the rays ; but Kepler WaS wrong in supposing that the rays which were tangents to the drop reached the eyc of the observer ; for if this were the case, the diameter of the bow, as Montucla has remarked, would be only 14° 24'.
A happy experiment seems to have at last suggested the true theory of the rainbow to a writer of very slen der attainments. Mark Antony dc Dominis, archbishop of Spnlatro, in a treatise De 1?adiis Visua et Lucia, published in 1611, by lidrtolus, maintains that onc re flexion and two refractions in the drop of water, NVCI'C suffi( ient to bring thc rays that formed the bow to the eye of the spectator. 'Phis explanation was either ve rifled or suggested by a globe of glass, filled with wa ter, which he viewed when exposed to the solar ray, under the same circumstances as the drops of rain. In this way lie found that the colours were the same, and arranged in the same order as in the natural bow. De Dominis supposed that the red rays were those which emerged nearest to the bottom of the drop, and that, having traversed the least quantity of water, they thus preserved a greater intensity than thc green and blue rays, which, emerging at a part of the drop most remote from the bottom, passed through a greater quantity of water, and were therefore less brilliant. He likewise remarked, that all the rays of the sante colour must emerge from the drop of water at a part similarly situated with regard to the bottom of the drop centrally opposite to thc sun, and that they therefore ought to form equal angles with a line drawn from the sun, through the eye of the spectator. Hence it follows that the zones of colour should appear circular ; and as the red rays form a greater angle with the above line, they ought to appear more elevated, and the red zone ought to be the exterior one. The vagueness and general inac curacy of the reasoning of De Dominis has led some French authors to deprive him of all that merit which both AIontucla and Priestley have concurred in assign ing to him His explanation of the interior bow, it is true, is sufficiently absurd; and his observations on vi sion and colours arc of the most crude kind ; but what ever be his errors, and however vague and indefinite be his explanations, it is impossible for any philosopher, free from the influence of national partiality, to deny that the Italian prelate has given such an explanation of the general phenomena of the exterior bow, that any other philosopher of more optical knowledge and of in fcrior acuteness could nnt fail, without any stretch of intellect, to give precision and perfection to the expla. nation. This unfortunate author, who had risen to such high ecclesiastical honours, had the imprudence to ex press sentiments inconsistent with his situation, and was obliged to leave Italy, and take refuge, in the year 1616, in London, where he spent some years. Being induced by various promises to return to Italy, his in discretion again involved him in new troubles, and hav ing been arrested by Pope Urban VIII. he was impri soned in the Inquisition, where he died of poison.* If the telescope was not actually invented by Roger Bacon, or by Baptista Porta, the science of optics had now attained that degree ot maturity, when its inven tion. as well as that of the microscope, was almost in evitable. If any of these valuable instruments bad been the unequivocal invention of a single individual, at a time when they were utterly unknown, the remarkable effccts which they produced must have excited univer sal attention, and secured to the inventor a reputation of the highest °icier. It is a remarkable fact, howtver, that no well founded claim to the invention of the tele scope has yet been made out ; and the number of com petitors for this honour affords the most unequivocal evidence that the telescope was brought into the condi tion of a portable and efficient instrument by steps so gradual, that no individual had any real claim to be re garded as its inventor.