292 W. LeConte Stevens—Notes on Physiological Opties. 
rating it in refutation of Brewster’s theory, was probably due 
to the fact that the fallacy and popularity of the latter, in its 
application to the stereoscope, had not been brought especially 
to his attention. 
2. ReLatTioN BETWEEN DtrrErenr ELEMENTS oF BrNnocuLaR 
PERSPECTIVE. 
The fact that in all stereoscopic vision there is necessarily an 
interruption of the usual relation between the axial and focal 
adjustments of the eyes was first noticed by Professor W. B. 
ogers, who makes however no reference to the production of 
any disturbance of perspective, as noticed by myself. In the 
articles already published it has been shown that even when 
the stereograph is so constructed as to exclude to the utmost 
the ordinary elements of perspective, there are left still three 
to consider. These are— 
I. The optic angle, positive or negative, enclosed by the 
visual lines and interpreted through the sensation of contraction 
or relaxation in the rectus muscles of the eyeballs. . 
II. The focal adjustment, interpreted through the sensation 
of contraction or relaxation in the ciliary muscle encircling the 
crystalline lens. 
III. The visual angle, subtended by the diameter of the ob- 
ject regarded, and interpreted by recognition of the retinal 
area impressed but instantly and unconsciously referred to the 
external objec 
ith a view to finding, if possible, what relation these three 
elements bear to each other in abnormal vision like that in the 
stereoseope, I constructed a modification of the instrument 
c 
originally devised by Wheatstone. Upon a cubical block, 
‘\ 
x 
al 
B 
(fig. 1) two plane mirrors, m and m’, were cemented, and a pair 
¢ 
. 
VD’ 
ne 
of arms were attached to carry the conjugate pictures, A and 
