10 
MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
Daguerreotypes of small dimensions, the instrument represented, fig. 4, shortened in 
its length from 8 inches to 5, and lenses of 5 inches focal distance placed before and 
close to the prisms. 
§ 22 . 
I now proceed to another subject — to the consideration of those phenomena which 
I have termed Conversions of Relief. 
In § 5 of my first memoir I noticed the remarkable circumstance, that when the 
drawing intended to be seen by the right eye is presented to the left eye in the 
stereoscope, and vice versd, a totally different solid figure is perceived to that seen 
before the transposition. I called this the converse figure, and showed that it differs 
from the normal figure in the circumstance, that those points which appear the most 
distant in the latter, appear the nearest in the former. 
The pictures being, in the first place, presented directly to their corresponding 
eyes, as in the refracting stereoscope, and exhibiting therefore the resultant image in 
its normal relief, the conversion of the relief may be effected in three different ways, — 
1st, by transposing the pictures from one eye to the other, as mentioned above ; 
2ndly, by reflecting the pictures, while they remain presented to the same eye, as in 
the reflecting stereoscope; and Srdly, by inverting the position of the pictures with- 
out transposing them. 
The following considerations will explain the cause of the conversion of relief in 
the preceding cases. 
If two different objects, or parts of an object (fig. 5 a), have a greater lateral distance 
between them on the right-hand picture than that which they have on the left-hand 
picture, the optic axes must converge more to make the left-hand than to make the 
right-hand objects coincide, and the left-hand object will appear the nearest. 
If the pictures be now transposed from one eye to the other (fig. 5 a'), the greatest 
distance will be between the corresponding points of the picture presented to the 
left eye ; the optic axes must therefore converge less to make the left-hand objects 
coincide, and tlie right-hand object will appear the nearest. 
If the pictures, remaining untransposed, be each separately reflected (fig. 5 b), the 
relative distances of the corresponding objects remain the same to each eye, and the 
left-hand object will still appear nearest ; but in consequence of the lateral inversion 
of the objects in each picture by reflexion, that which was previously on the left will 
now be on the right, and therefore, the object which before appeared nearest, will 
now appear farthest. 
When the pictures are turned upside down, still remaining untransposed (fig 5 c), 
the objects are reversed with respect to the right and left, in the same manner as 
they are when reflected, and the lateral distances between the objects remaining the 
same to each eye, precisely the same conversion of relief is produced as in the pre- 
ceding case, except that the resultant image is inverted. The diagram (fig. 5) repre- 
