Barrett—On Entoptic Vision. 8 
hole aperture and the shortest eye-cups should be used, in order to 
give a large retinal illumination, and the pillar of the Entoptiscope 
should be fixed vertically, so that the eye looks directly downwards. 
The muscze will now be seen nearly stationary, for being rather 
lighter than the vitreous humour they will slowly ascend, and thus 
their motion being in the line of sight their shadows will not be 
displaced, but become clearer as they approach the retina. 
Dr. Jago, in his “‘ Entoptics,” gives some careful drawings he made 
of different types of musce. Helmholtz, in his ‘“ Physiological 
Optics,” also gives some excellent drawings of the musce, and 
divides them into four or five groups according to their appearance. 
More recent writers have also depicted and investigated the muscee, 
so that it is needless to dwell further on this subject. In my own 
case, as I mentioned in Part I., footnote on p. 56, the muscz have 
increased in number, size, and persistency since the development 
of cataract in my eyes, and are constantly seen when looked for 
in the right eye, where the cataract is more advanced. 
(8). The Entoptiscope also enables the ‘‘light-sense’’ of an obser- 
ver to be tested, and the threshold of visibility of each eye deter- 
mined, 7.e. the degree of illumination which forms the lowest limit 
of visibility—the /ux-liminal point, it might be termed. For this 
purpose the smallest aperture may be used in the case of a normal 
eye, or larger, in persons suffering from cataract, and a small, 
steady source of light (such as a night-light) is shifted to different 
distances from the mirror of the instrument in a well-darkened 
room. The limiting distance is then measured, and, as the illu- 
mination of the ground-glass stage follows the law of inverse 
squares, the light threshold is at once found, relatively to some 
standard. The ‘‘adaptation-time,” or rise of sensibility when 
in darkness, of the eye of the observer is thus also accurately 
found ; further the visual acuity and colour-sense, under different 
degrees of illumination, can be conveniently examined in this way. 
(9). As was explained in Part I., § 5, a minute object can be 
clearly seen when held close to the eye and viewed through a 
stenopic screen. Under such circumstances a highly magnified 
image of the object is perceived from the large visual angle sub- 
tended by the object, whilst the blurring of the image on the 
retina is prevented owing to the circles of diffusion being reduced 
