W. LeConte Stevens—Notes on Physiological Optics. 299 
The attachment consists of an ordinary cross-bar, MN, fig. 4, 
which may be placed as near as convenient in front of the eyes 
whose optic centers are at O and O’, the visual lines being par- 
allel and passing through points C and OC’. These are the cen- 
ters of the conjugate series of concentric circles, on cards whose 
planes are perpendicular to the principal plane of vision, and 
which rest on extra short bars, PQand P’Q’. The latter are 
pivoted on the cross-bar so as to revolve about vertical axes 
passing through C and C’ respectively. Let ED and E’D’ be 
the horizontal diameters of the largest circles, the cards having 
been revolved so as to make with each other a dihedral angle 
opening toward the observer. Their relation to the visual lines 
's obviously the same as if their planes were coincident and the 
visual lines crossed, making the optic angle equal to the sum of 
NCD and MC’E’, as in Brewster's experiment. The retinal 
projections of ECD and E’C’D’ are ecd and e’e'd’. Since the 
t 
ured by the difference of the angles DOC and D’O’C’. The 
associated contraction of the external rectus muscles which this 
necessitates at once produces the sensation that habitually ac- 
companies recession of the object binocularly viewed. The 
0 
ing on the extent of the minor axis in each. If the successive 
vertices be connected, we have two curved lines, ACB and 
A’C'B’. If these be binocularly combined and externally pro- 
Jected, since CC’ is less than A A’ and BB’, optic divergence 
the observer. 
Let F and G (fig. 5) be points symmetrically situated with 
regard to the vertical diameter and hence equidistant from D 
Am. Jour, oe Serres, Vou. XXIL, No. 136.—AprRIL, 
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