468 SPECIAL PHYSIOLOGY. 



greater, in proportion to the intensity of the light which produces 

 them. Gray spots occupying a white surface, present a darker ap- 

 pearance than a gray ground of the same tint. The production of the 

 physiological colors by contrast, as just mentioned, is a further ex- 

 ample of this class of phenomena. A strip of gray paper placed on a 

 bright colored field, presents a faint tint of the color which is the op- 

 posite of that of the surface on which it lies ; on a red surface, it fre- 

 quently has a greenish tint ; on a green surface, a reddish tint, and so 

 on. Such phenomena are only produced, when the surface of the 

 field is of a very bright color, and when the portion of the retina ex- 

 posed to the action of the new color, is in a state of relative repose 

 (Muller). 



When, by means of the stereoscope, two different colors are thrown 

 upon corresponding points of the retina, the impressions sometimes 

 alternate; sometimes one color preponderates; at other times one color 

 appears in one part of the visual field, and the other in the other part ; 

 and, lastly, the two colors may be blended into a mixed or compound 

 color. These curious experiments prove that the impressions are more 

 or less blended in the sensorium. The blended colors are usually very 

 bright. On looking with one eye through a colored glass at the sky, 

 and keeping the other eye closed, or looking with it at the sky without 

 a glass, it is found that in the former eye, there arise spectra of the 

 complementary color to that of the glass, in the latter, spectra of the 

 same color as that of the colored glass. The former eye is sufficiently 

 excited to produce secondary or negative spectra, whilst the latter, 

 less perfectly stimulated, but still affected through the color in the 

 other eye, only produces positive spectra. 



Irritation or congestion of the retina, altered conditions of the optic 

 sensorium, diseases of the brain, dreams, and peculiar mental states, 

 give rise to various kinds of spectral phenomena. Bodies seated in, 

 or on, the eye, also produce appearances in the field of vision, called 

 entoptical images, such as motes, or muscce. Thus, fixed particles of 

 blood, lymph, or pigment, on the retina, or others in the lens, such as 

 radiated streaks or spots, cause fixed muscce; and movable particles, 

 on the surface of the cornea, such as tears or mucus, in the aqueous 

 humor, or in a softened vitreous humor, cause muscce volitantes, or 

 flying muscce. They are of various forms, some looking like spots or 

 streaks, others assuming a hair- or bead-like shape. 



Such of these entoptical images as are caused by objects in front of 

 the retina, are named extra-retinal. They are not usually noticed, 

 because light passes behind the little objects, which would otherwise 

 cast a shadow on the retina ; but by admitting the light into the eye, 

 through a minute hole in the card, so that the retina receives rays 

 from one direction only, distinct shadows of such objects are cast on 

 the retina, and produce the entoptical images. If the orifice in the 

 card be in the principal anterior focus of the eye, that is, about half 

 an inch in front of it, the entering luminous rays become parallel 

 behind the lens, and the images are of the same size as the object; if 

 the card is nearer the eye, the rays diverge, and the shadows are 

 larger ; if it is moved further off, the rays converge, and the images 



