RECEPTOR ORGANS 



519 



met with his name in connection with several other fundamental phenomena, such 



as the doctrine of energy, the electrical double-layer, the rate of the nerve impulse, 



and so on, this is perhaps the most appropriate place to call attention to his 



portrait, which will be found in Fig. 163 (given by the kindness of Dr J. T. 



Bottomley, of Glasgow). His two books on "Physiological Optics" and on 



"Sensations of Tone" 



remain the standard 



works on the subjects 



with which they deal. 



A third edition of the 



former has been brought 



out (1909-1910) by 



Gullstrand, von Kries, 



and Nagel. 



The function of the 

 dioptric system of the 

 eye is essentially 'a 

 question of geometrical 

 optics. It can be satis- 

 factorily treated by the 

 Gauss method of reduc- 

 tion to certain refract- 

 ing surfaces at definite 

 distances apart. Details 

 may be found in the 

 textbooks; that of 

 Parsons (1901) and the 

 article by von Rohr 

 (1909) may be men- 

 tioned. We must pass 

 on to the consideration 

 of the phenomena which 

 have been found to 

 occur in the retina in 

 response to stimulation 

 by light. 



Movements of Cones. 

 Slow movements of 

 the cones in the frog, 

 brought about by con- 

 traction of the long 

 fibres attached to their 

 bases, were described 

 by Van Genderen Stort 

 (1887) (see Fig. 613, 

 p. 522, of Schiifer's 

 " Essentials of Histol- 

 ogy "). They appear 

 to result from a reflex, 

 since light entering the 

 other eye causes retrac- 

 tion of the cones in the 

 eye which has not been exposed to light. The effect is also produced by light on 

 the skin, by injection of strychnine and by local electrical stimulation. It is 

 difficult to see what is the function of this movement. It has been suggested 

 that it may be a relic of an ancestral state in which the photo receptive cells of 

 the epidermis were connected directly to contractile cells, although, if we accept 

 the view of the origin of the eye in nerve centres, there are obvious difficulties in 

 t^his interpretation. 



FIG. 163. 



PORTRAIT OF HELMHOLTZ. Taken in his 

 laboratory on 7th July 1894. 



(By kind permission of Dr J. T. Bottomley.) 



