346 W. LeConte Stevens—Physiological Optics. 
ArT. XXX VII.—WNotes on Physiological ‘etal No. IV; by 
W. LeConte STEVEN 
1. On Votuntary Conrron or Foca ACCOMMODATION, 
In my last paper the visual effect of associated muscular 
action of the two eyes was discussed, the investigation having 
led to the discovery of a new mode of stereoscopy. Hxperi- 
ments have also been made on the effect of muscular action in 
a single eye. 
If the axial adjustment and visual angle be kept as nearly as 
possible unchanged while the gaze is directed upon a luminous 
surface, such as the globe of a gas lamp, it is very easy to 
throw the crystalline lens of each eye out of focus, acecommo- 
dating for a nearer point, so that the object appears blurred in 
consequence of the production of diffusion circles on the reti- 
I find it possible in this way to exercise extreme contrac- 
tion of the ciliary muscle at will. It is impossible to avoid 
slight associated Sct o of the internal rectus muscles, but 
this may be controlled to such an extent that the only effect 
noticeable is i of one eye, while the other is directed 
to some previously clearly defined object. The effect is first 
an encroachment of diffusion circles upon the surrounding reti- 
nal area as well as upon the image itself, producing dimness of 
vision and hence apparent enlargement ‘of the object which is 
necessarily ill-defined. Butas the ciliary contraction increases, 
the object apparently ditnintahent in size to a marked extent, sorme- 
times eithoat to half its Bynes? area, while the halo due to diffu- 
sion circles does not widen, but on the contrary grows narrower. 
This is due to the contraction of the pupil, the effect being that 
of using a new stop in front of the lens, as it thickens and ac- 
quires more ‘‘depth of focus,” while its theoretic focal length 
is less. The pupillary contraction can be noted by an assistant 
and approximate measures be made of the change in diameter 
of the opening. This pupillary change very quickly follows 
the contraction of the ciliary muscle. The area of my bia 
has thus been repeatedly diminished, within two secon 
time, from 12 sq. mm. to 12 sq.mm. The estimated distance 
of the object is very uncertain. It may be judged nearer be- 
e 
peca dimness 
only perfently clear gtag seta therefore, is that of marked de- 
decrease in area, far greater than can be referred to mere 
encoachment of diffusion aioled. The distance between retina 
and lens remains sensibly constant, hence the perception is an 
illusion due mainly to abnormal muscular conditions. 
