522 Prof. Wheatstone on the Physiology of Vision. 



1st. Two plane mirrors are placed together so as to form a 

 very obtuse angle towards the eyes of the observer ; immediately 

 before them the object is to be placed at such distance that a 

 reflected image shall appear in each mirror. The eyes being 

 placed before and a little above the object, must be caused to 

 converge to a point between the object and the mirrors ; the 

 right-hand image of the left eye will then unite with the left- 

 hand image of the right eye, and the converse relief will be per- 

 ceived. The disadvantages of this method are that only parti- 

 cular objects can be examined, and it requires a painful adapta- 

 tion of the eye to distinct vision. 



2ndly. Place between the object and each eye a lens of small 

 focal distance, and adjust the distances of tbe object and the 

 lenses so that distinct inverted images of the object shall be seen 

 by each eye ; on directing the eyes to the place of the object, the 

 two images will unite, and the converse relief be perceived. 

 As the rays of light proceeding from the images have a greater 

 divergence than those which would proceed from the point to 

 which the optic axes are directed, long-sighted persons will see 

 the binocular image more distinctly by wearing a pair of short- 

 sighted spectacles. In this experiment the field of view is very 

 small, on account of the distance at which it is necessary to 

 place the lenses from the eyes ; but I have been enabled in this 

 manner to see beautifully the converse relief of a small ivory 

 bust and of other small objects, which, however, should be in- 

 verted in order to see them direct. 



3rdly. The inverted images of the lenses, instead of being 

 received immediately by the eyes, as just described, may be 

 thrown on a plate of ground glass, as in the case of the ordinary 

 camera-obscura, and may be then caused to unite by the means 

 employed in any form of the refracting stereoscope. 



§24. 



The cases of the conversion of relief when the object is regarded 

 with one eye only, some of which were known more than a cen- 

 tury ago, were taken into consideration and endeavoured to be 

 explained, by me in § 1 1 of the first part of this memoir, and 

 Sir David Brewster* has published some interesting and in- 

 structive observations on the same subject ; I will therefore not 

 revert to this matter here, but only to say that I have myself 

 never observed the conversion of relief when looking with both 

 eyes immediately on a solid object, and if it has been observed 

 by others under such circumstances, I should be inclined to 

 attribute the effect to an inequality in the impressions on the 



* Transactions of the Royal Society of Edinburgh, vol. xv. p. 365 &657. 



