Barrett—On Entoptic Vision, 83 
required to be held below the aperture in order to restore clear 
vision; the refraction by the cornea is thus seen to be far greater 
than that by the crystalline (23 times) ; this is owing to the latter 
being immersed in media only slightly differing from itself in 
refractive power. (0) Keeping the selected convex lens before the 
aperture, note that accommodation still takes place. (ce) Changing 
the aperture to the smallest size, note that clear vision is now 
restored without the use ot any external glass lens.’ (d) Note, also, 
that ocular parallax (§ 8) still remains, though the large corneal 
refraction is abolished. 
Immersion of the cornea in water also enables the observer, 
if astigmatic, to note whether any residual astigmatism is due to 
the crystalline lens, or whether any power of astigmatic accom- 
modation by the lens can take place, as some think does occur. 
Doubtless other applications of the Entoptiscope will occur to 
the ophthalmologist. One of the simplest is its use in testing 
astigmatism by the appearance presented by the luminous point, 
when the eye of the observer is gradually removed from, say, 3 to 
30 inches. (See Part III. (2).) The observer will notice in this 
experiment how the slightest pressure on the cornea completely 
alters the appearance of the elliptical diffusion-spot, or other image, 
which the luminous point assumes in astigmatic eyes. T'scherning 
goes so far as to say there is no optic defect which is not shown 
by means of the figures presented by a luminous point. 
Note.—I have to thank my friend Dr. C. E. FitzGerald for pointing out 
to me that Snellen and Landolt, in their classical Handbuch der Augenheilkunde 
published in 1874, vol. 3, p. 178, were the first to speak of the great 
importance of entoptic examination in observing the development of cataract. 
They add the proviso ‘‘if the patient is sufficiently intelligent.” It is true 
all entoptic examination requires a certain degree of intelligence on the part 
of the observer ; but I have found that a very little instruction enables any 
one, not hopelessly stupid, to use the Entoptiscope quite successfully. 
1 For my own part I cannot understand why evolutionary processes did not lead to 
increased sensitiveness of the retina and a pin-hole eye like that of the nautilus, with a 
projecting cover. Such an eye would have many advantages over our own; no 
accommodation and no spectacles would be required; no spherical or chromatic 
aberration would exist, and no cataract would occur. And how did the lens originate ? 
Only in its perfect state would it be of use; and this, we assume, could only have been 
the result of imperfect stages which would be useless. 
