BINOCULAR VISION. 31 



he allowed light to fall on it, after passing through the fresh lens of an 

 ox's eye. The result was, that instead of being deeply coloured, as it 

 would have been by ordinary diffused light, it scarcely underwent any 

 change. The experiment was then performed with the cornea as the 

 transparent medium, then with the vitreous humour, and lastly with all 

 the three media together. And the general results which he obtained 

 were, that the lens, instead of allowing the chemical rays to pass through 

 it, absorbs them very largely ; that the cornea and the vitreous humour 

 absorb them also, though in a less degree ; and that with the three media 

 together, the absorption is almost complete. In a more recent set of 

 experiments,* he obtained equally satisfactory results by employing 

 photographic paper as the sensitive surface on which the rays of light 

 were allowed to fall after traversing the transparent media of the eye. 

 The results of these latter experiments proved, at least, that those rays 

 situated to the outside of the violet are arrested, although the paper was 

 deeply blackened by the violet ray itself. 



To determine the same point in regard to the calorific rays, M. Briicke 

 made use of a thermo-electric apparatus, and on allowing light to fall on 

 this through the transparent media of the eye, he observed that its needle 

 underwent no change ; shewing, therefore, that very little, if any, of the 

 calorific portion of a ray of light is transmitted through the eye. 



Binocular vision. A somewhat different explanation of the mode in 

 which the reflection of two different views of a solid object in the 

 stereoscope \ produces in the mind the idea of a single object similar 

 to the original, has been proposed by Tourtual,J who details the results 

 of numerous experiments in support of it. But in this explanation 

 there appears to be little really at variance with the one afforded by 

 Professor Wheatstone, and in the results of the experiments nothing 

 which can invalidate the main conclusion derived from Professor 

 Wheatstone's philosophical researches, namely, that our conviction of 

 the solidity of an object, or of its projection in relief, is due, in great 

 measure, to the circumstance of corresponding portions of the two 

 retinae receiving the impression of a different view of the object the 

 one view as seen by the right eye, the other by the left whereby an 

 exact counter-part of the original is produced. Tourtual objects to this 

 view, and considers that the fact of the solidity of a near object being 

 distinctly realized by one eye alone, affords a conclusive proof that the 

 visual perception of an object of three dimensions, is an operation of the 

 mind, and is not necessarily dependent on the formation of two images of 

 this object on the retinse. Volkmann and Briicke also appear to agree 

 with Tourtual in the opinion that the perception of a solid object in the 



* Op. Cit. 1846, p. 379. t Mailer's Physiology, p. 1205. 



J Report in Mliller'a Archiv. 1842. 



