Barretr—On Entoptic Vision. ae 
can be thus set at rest at once. Retinal defects, if existing, are 
of course still seen, but are easily distinguished from obscurities 
in the eyeball, and can be examined by retinoscopy. 
§ 6. 
In order to find the eract position within the eyeball of any 
obscurity, two methods were fully described in Part I. The best 
method is that of employing two minute and closely- -adjacent 
apertures i in the diaphragm ; two overlapping images of the pupil 
are thus produced, together with duplicated shadows of the obscuri- 
ties. As was fully explained in Part I., §9, when the path of the 
rays within the eye is parallel, the distance apart S of the 
duplicated shadow is in the same ratio to the distance R of the 
entoptic object from the retina as the distance apart C of the 
centres of the two overlapping pupillary discs is to the distance P 
of the pupil from the retina, or 
S 
Ji = a v2) 
The distance Pin a normal eyeis 19mm. As C corresponds 
to the portion of the circles which do noé overlap, a comparison of 
that distance with S can at once be made. If the entoptie object 
be near, or on the cornea, its duplicate shadow SS is seen further 
apart than C’; if it be on the anterior face of the crystalline, SS = C; 
if on the posterior face, SS is rather smaller than CO; if near the 
retina, SS is seen well within the overlapping part of the circles, 
and much smaller than C; in this case the duplicated shadows 
SS are, in fact, quite close together. 
Fig. 7, p. 74, isa careful tracing I have made of the obscurities 
seen in my left eye with a double aperture; the distance C 
corresponds to the distance apart of the centres of the two circles. 
If the reader will take the trouble to measure the distance asunder 
of any of the prominent duplicate obscurities, he will find that 
distance exactly equal to C’; hence these opacities lie on the anterior 
face of the lens in the pupillary plane. A small musca near the 
retina is shown, with its double shadow close together at m. Owing 
to the superposition of the two dises of light, the overlapping part 
