THE SENSE OF VISION. 331 



as it contains no rods, is destitute of visual purple, makes it impossible to 

 regard this coloring matter as essential to vision. The most probable theory 

 of its function is perhaps that which connects it with the adaptation of the 

 eye to varying amounts of light, as described on p. 326. 



In addition to the above-mentioned movements of the pigment epithelium 

 cells under the influence of light, certain changes in the retinal cones of frogs 

 and fishes have been observed. 1 The change consists in a shortening and thick- 

 ening of the inner portion of the cones when illuminated, but the relation of 

 the phenomenon to vision has not been explained. 



Like most of the living tissues of the body, the retina is the seat of electri- 

 cal currents. In repose the fibres of the optic nerve are said to be positive in 

 relation to the layer of rods and cones. When light falls upon the retina this 

 current is at first increased and then diminished in intensity. 



Sensation of Light. — Whatever view may be adopted with regard to the 

 mechanism by which light is enabled to become a stimulus for the optic nerve, 

 the fundamental fact remains that the retina (and in all probability the layer 

 of rods and cones in the retina) alone supplies the conditions under which this 

 transformation of energy is possible. But in accordance with the " law of 

 specific energy " a sensation of light may be produced in whatever way the 

 optic nerve be stimulated, for a stimulus reaching the visual centres through 

 the optic nerve is interpreted as a visual sensation, in the same way that 

 pressure on a nerve caused by the contracting cicatrix of an amputated leg 

 often causes a painful sensation which is referred to the lost toes to which the 

 nerve was formerly distributed. Thus local pressure on the eyeball by stimu- 

 lating the underlying retina causes luminous sensations, already described as 

 " phosphenes," and electrical stimulation of the eye as a whole or of the stump 

 of the optic nerve after the removal of the eye is found to give rise to sensa- 

 tions of light. 



Vibrations of the luminiferous ether constitute, however, the normal stim- 

 ulus of the retina, and we must now endeavor to analyze the sensation thus 

 produced. In the first place, it must be borne in mind that the so-called ether 

 waves differ among themselves very widely in regard to their rate of oscilla- 

 tion. The slowest known vibrations of the ether molecules have a frequency 

 of about 107,000,000,000,000 in a second, and the fastest a rate of about 

 40,000,000,000,000,000 in a second — a range, expressed in musical terms, of 

 about eight and one-half octaves. All these ether waves are capable of warm- 

 ing bodies upon which they strike and of breaking up certain chemical com- 

 binations, the slowly vibrating waves being especially adapted to produce the 

 former and the rapidly vibrating ones the latter effect. Certain waves of 

 intermediate rates of oscillation — viz. those ranging between 392,000,000,- 

 000,000 and 757,000,000,000,000 in a second — not only produce thermic and 

 chemical effects, but have the power, when they strike the retina, of causing 

 changes in the layer of rods and cone-, which, in their turn, act :i- a stimulus 

 to the optic nerve. The ether waves which produce these various phenomena 

 are often spoken of as heat rays, light rays, and actinic or chemical rays, but 

 1 Kngelmann : Archiv fur die gesammte Physiologic, xxxv. 498. 



