J. D. Dana—“Kames” of the Connecticut River Valley. 451 
tual images by causing rays from two pictures to deviate and 
appear to come from one central combined external picture. 
cause the stereoscopic displacement to be constant for projec- 
tions of the points where these lines cut the foreground and 
background planes. This fact alone is enough to suggest that 
in vision through the stereoscope the midpoint between the 
eyes must be the point of origin from which distance and direc- 
tion are to be perceived. A truth that was first recognized as 
a physiological necessity is thus confirmed by purely mathe- 
matical considerations. 
he dissociation between focal and axial adjustments in 
forced convergence or divergence is at first troublesome and 
n 
measure after a little practice in ocular gymnastics. If the 
eyes are Sotntctiatle we are apt to forget that the vision is 
abnormal, and to assume that conditions exist which belong 
only to niiFiaal vision. To ascertain what modifications are 
imposed by physiological conditions upon the generally ac: 
cepted mathematical theory of the stereoscope has been the 
chief object of the present investigation. 
New York, Sept. 16, 1881. 
Art. LIX.— On the relation of the so-called “‘ Kames” of the 
ante River Valley to the Terrace-formation; by JAMES 
D 
eology of New Hampshire, Part III, Chapter i, Modified Drift in New 
‘oases by Warren Upham, pp. 3-177. 1878. A synopsis of Mr. Upham’s 
Report, by its author, was published i in this Sona vol. xiv, p. 459, 1877, 
