Prof. Wheatstone on the Physiology of Vision. 461 



"The covering or tunic of the ovisac; and about the same 

 time, the peculiar granules of the ovisac arrange them- 

 (, J selves to form, 



{The tunica granulosa, 

 The retinacula, and 

 The membrana granulosa. 



Such of these structures as are present in the ovary of other Ver- 

 tebrata, appear to originate in the same order as to time. 



" Contributions to the Physiology of Vision." By Charles Wheat- 

 stone, Esq., F.R.S., Professor of Experimental Philosophy in King's 

 College, London. Part the First. " On some remarkable and hitherto 

 unobserved Phsenomena of Binocular Vision." 



The author first shows that the perspective projections of an ob- 

 ject upon the two retinae dilFer according to the distance at which 

 the object is placed before the eyes ; if it be placed so distant that 

 to view it the optic axes must be parallel, the two projections are 

 precisely similar ; but if it be placed so near that to regard it the op- 

 tic axes must converge, a different perspective projection is present^ 

 ed to each eye ; and these perspectives become more dissimilar as 

 the convergence of the optic axes becomes greater. Notwithstand- 

 ing this dissimilarity between the two pictures, which is in some 

 cases very great, the object is still seen single ; contrary to the very 

 prevalent metaphysical opinion, that the single appearance of ob- 

 jects seen by both eyes is owing to their pictures falling on corre- 

 sponding points of the two retinae. After establishing these prin- 

 ciples, the author proceeds to ascertain what would result from pre- 

 senting the two monocular perspectives, drawn on plane surfaces, to 

 the two eyes, so that they shall fall on the same parts of the two re- 

 tinae as the projections from the object itself would have fallen. Seve- 

 ral means are described by which this may be accomplished ; but the 

 author especially recommends for this purpose an apparatus called by 

 him a stereoscope, which enables the observer to view the resulting 

 appearances without altering the ordinary adaptation of the eyes, 

 and therefore without subjecting these organs to any strain or 

 fatigue. It consists of two plane mirrors with their backs inclined 

 to each other at an angle of 90°, near the faces of which the two 

 monocular pictures are so placed that their reflected images are seen 

 by the two eyes, one placed before each mirror, in the same place ; 

 the apparatus has various adjustments by means of which the mag- 

 nitude of the images on the retinae may be varied, and the optic 

 axes differently converged. If the two monocular pictures be thus 

 presented one to each eye, the mind will perceive, from their com- 

 bined effect, a figure of three dimensions, the exact counterpart of 

 the object from which the pictures were drawn ; to show that this 

 curious illusion does not in the least depend on shading or colouring, 

 the illustrations principally employed are simple outline figures, 

 which give for their perceived resultants skeleton forms of three di- 

 mensions. Each monocular outline figure is the representation of two 

 dissimilar skeleton forms, one being the form which it is intended to re- 

 present, and another, which Prof .Wheatstone calls its converse figure. 



