276 Animal Life and Intelligence. 



At what distance apart, on the most delicate part of the 

 retina, can two points of stimulation be recognized as dis- 

 tinct from each other ? If the points of stimulation be not 

 less than ^-^-^ of an inch ('004 millimetre) apart, they 

 can be distinguished as two. Below this they fuse into 

 one. The diameter of the end of a single cone in the 

 yellow spot is also about g-(/(jQ of an inch ('0045 millimetre). 



With regard to the mode in which the stimulation of 

 the retinal elements is effected, we have no complete know- 

 ledge. Certain observations of Boll and Kiihne, however, 

 show that when an animal is killed in the dark the retina 

 has a peculiar purple colour which is at once destroyed if 

 the retina be exposed to light. If a rabbit be killed at the 

 moment when the image, say, of a window, is formed on 

 the retina, and the membrane at once plunged in a solution 

 of alum, the image may be fixed, and an " optogram " of 

 the window may be seen on the retina. The discharge of 

 the colour of the retinal purple may be regarded as the 

 sign of a chemical change effected by the impact of the 

 light-vibrations. But in the yellow spot there seems to be 

 no visual purple. It is, indeed, developed only in the rods, 

 not in the cones. Here, probably, chemical or metabolic 

 changes occur without the obvious sign of the bleaching of 

 retinal purple. In the dusk-loving owl the retinal purple 

 is well developed, but in the bat it is said to be absent. 



We saw that in the case of hearing the auditory organ 

 is fitted to respond to air-borne vibrations varying from 

 about thirty to thirty thousand per second. And though 

 the details of the process are at present not well under- 

 stood, it is believed that certain parts of the recipient 

 surface are fitted to respond to low tones, other parts to 

 intermediate tones, and yet others to high tones. Thus 

 the reception is serial. If there be two pianos near each 

 other, accurately in tune, any note struck on one will set 

 the corresponding note vibrating in the other.* The 

 auditory organ may be likened to this second piano. 

 Special parts respond to special tones. 



* The dampers must, of course, be lifted by depressing the loud pedal. 



