SENSE OF SIGHT. 581 



The ophthalmoscope is used to detect changes in the retina, as 

 in Bright' s disease of the kidneys, and also for testing errors of 

 refraction, as in myopia and hypermetropia. For this latter 

 purpose skiascopy also is employed, which is defined as " a method 

 of determining the refraction of the eye by examining the move- 

 ments of light and shadow across the pupil when the retina is 

 illuminated by light thrown into the eye from a moving mirror." 



Light. The word " light" is used in two senses: 1. With 

 reference cO the sensation produced in the brain ; and 2. With 

 reference to the cause of that sensation. 



Light, the cause, is defined as " the form of radiant energy that 

 acts on the retina of the eye, and renders visible the objects from 

 which it comes ; the illumination or radiance that is apprehended 

 by the sense of vision" (Standard Dictionary). It is " a periodic 

 disturbance in a very subtle and highly elastic medium which is 

 supposed to exist everywhere in space, even pervading the inter- 

 molecular spaces in matter. This medium is known as the ether, 

 and vibrating disturbances in it give rise to all the phenomena of 

 radiant energy. These disturbances are propagated through it as 

 waves, not of compression and rarefaction, but more like those of 

 the rope, the direction of vibration being transverse to that of 

 propagation " (Carhart and Chute). The reference to " the rope " 

 is to an experiment of laying a soft-cotton rope, about 5 feet long, 

 on a floor, and then, holding one end of the rope in the hand, setting 

 up vibrations in it by a quick up-and-down movement of the hand. 



When these waves in the ether reach the retina they produce 

 the sensation of sight, and, as has been stated, the portion of the 

 retina which is sensitive to light is the layer of rods and cones. 

 Just how this is accomplished is not known. Various theories 

 have been advanced to explain it : (1) That the waves of light be- 

 come waves of heat and thus act as thermic stimuli to the rods and 

 cones ; (2) that the waves of light become waves of electricity, and 

 that the stimuli are electric; and (3) that these waves produce 

 certain chemical changes, so that the stimuli are chemical. The 

 first and second theories may be passed by with a mere men- 

 tion, and, although the third is far from proved, yet there are facts 

 which make the theory worthy of attention and continued investiga- 

 tion, as a result of which the true explanation may be forthcoming. 



In the outer portions of the rods of the retina is a pigment, 

 rhodopsin or visual purple, which, when the retina is exposed to 

 light, becomes red, then orange, then yellow, and finally fades 

 away. When the eye is exposed to light, the pigmented epithe- 

 lium of the retina sends pigmented processes between the rods 

 and cones, and this pigment, fuscin, forms visual purple again, 

 and this reappears in the rods. If the pigmentary layer is sepa- 

 rated from the other layers of the retina, the formation of the 

 rhodopsin, after it has been bleached by light, does not occur. If 

 an eye, after having been protected from the light for a con- 



