Prof. Wheatstone on the Physiology of Vision. 509 



the same distance from the eyes. The adaptation is, therefore, 

 not entirely dependent on the divergence of the rays of light 

 which proceed from the object regarded, but also, in some degree, 

 on the inclination of the optic axes. I have acquired by practice 

 considerable power of adjustment, or rather disadjustment, of 

 the eyes, and can, without having recourse to artificial means, 

 see the binocular picture distinctly when its perceived magnitude 

 is widely different. Those to whom such an effort is painful 

 may employ short-sighted spectacles to see the binocular picture 

 when the eyes converge within the limit of distinct vision for 

 the distance at which the pictures are placed ; and long-sighted 

 spectacles when the eyes converge beyond that limit, or become 

 parallel. 



There is a means of avoiding to a very considerable extent the 

 influence of the adjustment of the eyes, and thereby enabling 

 the pictures to be seen distinctly within the entire range of the 

 inclination of the optic axes. This is by looking at the reflected 

 images in the mirrors through two very minute apertures, not 

 larger than fine pin-holes, placed near each eye, and illuminating 

 the pictures by a very strong light ; sunshine in the middle of 

 the day answers the purpose very well. By this expedient the 

 divergence of the rays of light is greatly diminished, and the 

 adaptation of the eyes does not materially influence the result. 



§18. 



Leaving this subject, I will now revert to the stereoscope and 

 its effects. 



Since 1838 numerous modifications of the stereoscope have 

 occurred to me, and several ingenious arrangements have also 

 been proposed by Sir David Brewster and Professor Dove ; but 

 there is no form of the instrument which has so many advan- 

 tages for investigating the phenomena of binocular vision as the 

 original reflecting stereoscope. Pictures of any size may be 

 placed in it, and it admits of every kind of adjustment. 



I have constructed a very portable reflecting stereoscope which 

 is represented at fig. 3. The sides fold over the mirrors, and 

 the mirrors then fold into a box, which is not larger than six 

 inches in any of its dimensions. To avoid the second feeble 

 reflexion from the anterior surface of the silvered glass, which 

 has a bad effect when the attention is attracted to it, I have 

 sometimes employed reflecting prisma. The reflecting surfaces 

 of the prisms should be silvered in order to obviate the unequal 

 brightness of the field of view on each side of the limit of total 

 reflexion ; and as it would be too costly to employ very large 

 prisms, they should have an adjustment to accommodate their 

 distance to the width between the eyes of the observer. 



