294 W. LeConte Stevens—Notes on Physiological Optics. 
produces the impression of continued recession in a positive 
direction. The visual angle has not been changed, and the 
focal adjustment not enough so to produce any very perceptible 
decrease in distinctness of vision. The image appears perhaps 
60™ or 70™ distant, but this estimate is quite uncertain. e 
_ apparent diameter is of course increased. The effect of constancy 
in the visual angle in this case seems to be the preponderating 
element in determining the judgment. 
The results of experiment with the apparatus just described 
are given in the curve A A’, of Fig. 2. The stereograph em- 
2. 
ran Oe 3 
A: 
oo: 
Ne Le ay 
sy ne 
“NY 
gee morn ped AS 
ze. BD) 
> Ht A Boe IP So? ae? abe “lp? 
focaiin of relief. Distances were estimated to the edge of 
the concavity, which was surrounded with a uniform black 
surface. Itis seen that the curve of theoretic distances, DD’, 
is cut by that of apparent distances, AA’ not far from the 
i ao 7" 20%. 
n 
irregular order, recording each value, of which I remained 
ignorant, and at the same time recording my corresponding est- 
mate of distance. The curve has been constructed from the 
record of six independent series of estimates. In consequence 
of the difficulty of securing perfect dissociation between axial 
and focal adjustments for large positive values of the optic 
error being +8™™. It is found almost to coincide with the the- 
oretic curve for a short distance on each side of the intersec- 
