574 ANIMAL PHYSIOLOGY. 
vious by looking at a flame through cobalt-blue glass, which 
allows only the red and blue rays to pass: the flame may appear 
red surrounded by blue or blue surrounded by red, according 
to the character of the accommodation of the eye at the time, 
Since the eye has to be accommodated for violet (see Fig. 415) 
more than blue, bodies of equal size, red in color, always appear 
nearer than violet ones. Hence, also, it is difficult to see the 
red and violet of the spectrum with equal distinctness at the 
same time. 
Entoptic Phenomena.—Opaque bodies in any of the media of 
the eye may cast shadows on the retina. 
When movable, as they often are in the vitreous humor, 
they are known as musce volitantes, from their fancied resem- 
blance to gnats. 
One looking through a microscope is apt at first to see what 
does not exist, apart from his own eye, owing to various forms 
of the nature now referred to, but which may be distinguished 
from real objects by the inability to fix them in the field of 
vision, for as soon as the attempt is made they vanish. 
Tears on the cornea and other inequalities from foreign 
bodies, pressure, etc., likewise give rise to such phenomena. 
An interesting little experiment, which illustrates both the 
alterations in size of one’s own pupils with the amount of light, 
and at the same time irregularities in their margins, if they 
exist, may be thus carried out: Let a pin-hole be pricked in a 
card, and, holding this close to the eye, look at a light or a 
bright surface. On opening and closing the other eye the 
changes in the size of the pupil of the first eye may be seen 
to alter with the amount of light admitted to the second—i. e., 
the field of view is alternately diminished and increased. 
Anomalies of Refraction.—1. We may speak of an eye in which 
the refractive power is such that, under the limitations referred 
to before (page 564), images are focused on the retina, as the 
emmetropic eye. The latter is illustrated by Fig. 416. In the 
upper figure, in which the eye is represented as passive (nega- 
tively accommodated), parallel rays—i.e., rays from objects 
distant more than about seventy yards (according to some 
writers much less)—are focused on the retina; but those from 
objects near at hand, the rays from which are divergent, are 
focused behind the retina. In the lower figure the lens 1s rep- 
resented as more bulging, from accommodation, as such diver- 
gent rays are properly focused. 
2. In the myopic (near-sighted) eye the parallel rays cross 
