72 Scientific Proceedings, Royal Dublin Society. 
Another and by no means unimportant use of the Entopti- 
scope by the oculist will be found in the certainty with which it 
will enable him to dispel alarm in very many cases where a 
patient, from perceiving some obscurities such as the musce 
volitantes in his field of view, imagines he is about to lose his 
sight. Looking through the Entoptiscope, if any musce are 
present, they will be conspicuously seen slowly moving over the 
illuminated stage; not only will their characteristic appearance 
and mobility enable the oculist to make an immediate diagnosis, 
but their actual size and position in the eyeball can be accurately 
determined. Fifty years ago, Sir David Brewster pointed out 
the value of entoptic observation in such cases. He remarks: 
“Few symptoms appear so alarming to nervous persons as the 
muse volitantes; and that these fears can be dispelled by the 
application of a recondite property of divergent light which has 
only been developed in our own day is one of the numerous 
proofs which the progress of knowledge is daily accumulating, 
that the most abstract and apparently transcendental truths in 
physical science will, sooner or later, add their tribute to supply 
human wants and alleviate human sufferings.” ! 
As the musce occur nearer the retina than the lens, their 
shadows can be seen, as is a matter of common experience, 
without the use of a homocentric pencil of rays. Hence a very 
simple method, which I referred to, but did not describe, in 
Part I., § 9, suggests itself for ascertaining approximately the 
relative position of any obscurities within the eyeball. This 
consists in gradually increasing the size of the aperture in the 
eye-piece of the Entoptiscope. Accordingly, a graduated series 
of orifices is made in the revolving diaphragm of the eye-piece. 
Upon rotating the diaphragm so that these orifices, from the 
minutest to the largest, successively come into view, the observer 
will note a corresponding disappearance of obscurities in the 
order of their distance from the retina. Finally, with the largest 
aperture, whilst the definite shadow of even advanced cataract is 
lost, the far smaller shadows of musce, or any obscurity very near 
the retina, still remain. The fears of a patient, if groundless, 
1 North British Review, November, 1856. 
