Chemistry and Physics. * 273 



from change and preserved as a white band in the midst of the dark- 

 ened paper. This # experiment was thought by the author strongly con- 

 firmatory of the view which he had taken, that of actinism, or the 

 chemical principle, and light, so far from being identical, are opposed 

 in action to each other, 



4. An Examination of Berkeley's Theory of Vision; by Sir 1>. 

 Brewster, (Proc. # Brit. Assoc, Athen., No. 1087.)— Sir D. Brews- 

 ter's attention was turned to this subject by having been called on to 

 give a course of lectures on the Philosophy of the Senses. He was 

 thus led most minutely to examine the well-known theory of vision by 

 Bishop Berkeley, and made himself minutely acquainted with its details. 

 He found, as he conceived, the fundamental principle, on which the 

 entire theory reposed, defective. Berkeley's first proposition, which 

 contained that principle, was that direct distance from the eye had no 

 linear representative in the image on the retina, and therefore that dis- 

 tance from the eye could not be a direct object of vision : now this 

 first proposition Sir D. Brewster conceived to be false in fact, and there- 

 fore the entire theory which was founded on it must be abandoned. 

 That there was a direct linear representative on the relina, even as 

 seen by a single eye, he showed by a diagram to be the result of the 

 rays entering the pupil ; and let the pupil be of what size it may, un- 

 less reduced to an actual point, which it never can be, it must allow 

 the rays to form an impression of the entire length of a line stretching 

 away directly from the eye, and therefore objects do not appear all at 

 the same distance from the eye, as would follow from the Bishop's the- 

 ory. But further, Dr. Berkeley founded his entire theory on an assum- 

 ed fact, which, if true at all, could be so only when one eye was used 

 alone. Man is not an animal with but one eye. An infant obtains his 

 first glances of the external world by opening on it both eyes ; and 

 from whatever sources its lessons are derived, both eyes must be ad- 

 mitted to be equally efficient in the knowledge conveyed. In deter- 

 mining this question, he did not consider it essential to decide how sin- 

 gle vision with two eyes was effected. It might be from the images 

 falling on corresponding points of the retina, — an opinion now exploded ; 

 or from the decussation of the filaments of the optic nerve, — an opin- 

 ion also exploded; or it might be the necessary result of the line of 

 visible direction — as it certainly was. But wheresoever it originated, 

 U was a fact which must be admitted. They had therefore to deter- 

 mine at what distance, and in what direction, a body would be seen by 

 two eyes when seen single; a determination greatly facilitated by that 

 beautiful instrument the stereoscope of his friend Professor Wheatstone. 

 •Sir David then, by reference to diagrams, explained how this deter- 

 mination was aided by the use of both eyes; and in confirmation of 

 Ins views appealed to 'facts recorded by naturalists, to prove that the 

 young of animals saw distances correctly almost at the instant of their 

 birth. The duckling ran to the water almost as soon as it broke the 

 shell. The young Boa constrictor would involve and bite an object pre- 

 sented to it; and in like manner no person ever saw a child use such 

 motions as proved it to perceive objects at its eye, to grasp at the sun or 

 rooon or other inaccessible object, but quite the contrary. He also con- 



Sfxond Series, Vol. VII, No. 20.— March, 184& 35 



