Barrett—On Entoptic Vision. 87 
others have discussed the subject fully.1_ The experiment is best 
made in a darkened room, and is readily explicable from the law 
that the stimulation in any way of a nerve-fibre excites only the 
sensation peculiar to that special group of nerves. 
This, however, is to some extent a subjective phenomenon ; 
and into purely subjective optical phenomena, such as the 
effects resulting from retinal fatigue, after-images, &c., I do 
not propose to enter. 
(7). Microscopists who have worked with high magnifying 
powers are aware that, with excessive magnification, a peculiar 
spotty appearance is produced in the field. This is due to the fact 
that, as the magnification increases, the image of the objective, from 
which proceeds the light that enters the eye, becomes smaller and 
smaller until it is practically a luminous point. Thus a homo- 
centric pencil of rays enters the eye; and the shadows of any dust 
on the eye-piece of the microscope, or of any obscurities on or 
within the eye, are thrown upon the retina along with the 
image of the microscopic object. Delicate microscopic details thus 
become indistinct owing to the fact that the very conditions which 
create high-power microscopy also create the conditions of entoptic 
vision. Helmholtz was the first to point this out in his paper on 
the “ Theoretical Limits of resolving Power in the Microscope.” 
The difficulty appeared to be insurmountable until Mr. J. W. 
Gordon entirely overcame it by receiving the luminous point of 
the image of the objective carrying with it the image of the 
microscopic object on a little ground-glass screen. This, by scat- 
tering the light, of course, destroys the homocentric nature of the 
pencil, and thus gets rid of the entoptic shadows. But the mag- 
nified grain of the ground-glass screen now becomes a serious 
objection ; this, however, Mr. Gordon entirely gets rid of by 
causing the little screen to be kept in rapid eccentric motion by a 
small electric motor. Under these circumstances eye-observation 
1 Optique Physiologique, pp. 266-280. See also Essai sur les phosphenes, par 
Dr. Serre. Paris, 1853, &c. 
* Die theoretische Grenze fiir die Leistungsfahigheit der Mikroscope, von H. Helmholtz, 
Poggendorff’s Annalen, 1874, p. 557. This important paper has been translated by 
Dr. Fripp, and is published in the Monthly Microscopical Journal, N.S., vol. xvi., 
p. 15, and in the Bristol Naturalists’ Society Proceedings, N.S., vol. i., part 3. 
