FROM ALHAZEN TO KE.PLBR.
An interval of nearly a thousand years divided Ptolemy from Alhazen, who, • in the history of optical discovery, appears as his immediate successor. This ingenious Ara bian lived in the eleventh century, and his merit can be more fairly, and will be more highly appreciated, now that the work of his predecessor has become known. The merit of his book on Optics was always admitted, but he was supposed to have borrowed much from Ptolemy, without acknowledging it ; and the prejudices enter tained in favour of a Greek author, especially of one who had been for so many years a legislator in science, gave a false impression, both of the genius and the in tegrity of his modern rival. The work of Alhazen is, nevertheless, in many respects, superior to that of Ptolemy, and in nothing more than in the geometry which it em ploys. The problem known by his name, to find- the point in a spherical speculum, at which a ray coming from one given point shall be reflected to another given point, is very well resolved in his book, though a problem of so much difficulty, that Montucla hazards the opinion, that no Arabian geometer was ever equal to the solution of it.' It is now certain, however, that the solution, from whatever quarter it came, was not borrowed from Ptolemy, in whose work no mention is made of any such question ; and it may very well. be doubted, whether, had this problem been proposed to him, the Greek would have appeared to as much advantage as the Arabian.
The account which the latter gives of the augmentation of the diameters of the hea venly bodies near the horizon his been already mentioned. He treated also of the re fraction of light by transparent bodies, and particularly of the atmospheric refraction, but not with the precision of Ptolemy, whose optical treatise Delambre seems to think it pro bable that he had never seen. The anatomical structure of the eye was known to him ; concerning the uses of the different parts he had only conjectures to offer ; but on seeing single with two eyes, he made this very important remark, that, when corresponding parts of the retina are affected, we perceive but one image.
Prolixity and want of method are the faults of Alhazen. Vitello, a learned Pole, commented on his works, and has very much improved their method and arrangement in a treatise published in MO. He has also treated more fully of the subject of re fraction, and reduced the results of his experiments into the form of a table exhibiting the angles of refraction corresponding to the angles of incidence, which he had tried in water and glass. It was not, however, till long after this period that the law which con nects these angles was discovered. The cause of refraction appeared to him to be the re sistance which the rays suffer in passing into the denser medium of water or glass, and one can see in reasoning an obscure idea of the resolution of forces. He also treats of the rainbow, and remarks, that the altitudes of the sun and bow together always amount to 42 degrees. He next considers the structure of the eye, of which he has given a toler
ably accurate description, and proves, as Alhazen had before done, 3 that vision is not performed by the emission of rays from the eye.
Roger Bacon, distinguished for pursuing the path of true philosophy in the midst of an age of ignorance and error, belongs to the same period ; and applied to the study of op tics with peculiar diligence. It does not appear, however, that he added much to the discoveries of Alhazen and Ptolemy, with whose writings, particularly those of the former, he seems to have been well acquainted. In some things he was much behind the Ara bian optician, as he supposed with the ancients that vision is performed by rays emitted from the eye. It must, however, be allowed; that the arguments employed on both sides of this question are so weak and inconclusive, as very much to diminish the merit of being right, and the demerit of being wrong. What is most to the credit of Bacon, is the near approach he appears to have made to the knowledge of lenses, and their use in assisting vi sion. Alhazen had remarked, that small objects, letters, for instance, viewed through a segment of a glass sphere, were seen magnified, and that it is the larger segment which magnifies the most. The spherical segment was supposed to be laid with its base on the letters, or other minute objects which were to be viewed. Bacon recommends the smaller segment, and observes, that the greater, though it magnify more, places the object farther _ off than its natural position, while the other brings it nearer. This shows sufficiently, that he knew how to trace the progress of the rays of light through a spherical transparent body, and understood, what was the thing least obvious, how to determine the place of the image. Smith, in his Optics, endeavours to show, that these conclusions were purely theoretical, and that Roger Bacon had never made any experiments with such glasses, notwithstanding that he speaks as if he had done so. This severe remark proceeds on some slight inaccuracy in Bacon's description, which, however, does not seem suf ficient to authorize so harsh a conclusion. The probability appears rather to be, as Molineux supposed, that Bacon had made experiments with such glasses, and was both practically and theoretically acquainted with their properties. At the same time, it must be acknowledged, that his credulity on many points, and his fondness for the marvellous, which, with every respect for his talents, it is impossible to deny, take away from the force of his testimony, except when it is very expressly given. However that may be in the present case, it is probable, that the knowledge of the true properties of these glasses, whether it was theoretical or practical, may have had a share in introducing the use of lenses, and in the invention of spectacles, which took place not long after.