The Senses of Afiimals. 



299 



the aether- vibrations. The figures are those given by 

 Professor Langley : — 



Wave-lengths in Number of 

 thousandths of vibrations per Effects on man. 

 a millimetre, second in billions. 



Qaality of radiations. 



Limit of photography, arti- 

 ficial source 



Limit of photography, solar 

 source 



Limit of violet to normal 

 eyes 



Limit of red to normal eyes 



Probable inferior limit of 

 temperature-sensations . . 



Longest waves hitherto re- 

 cognized with bolometer 



From this table it will be seen that, apart from the 

 possible extension of sight beyond human limits, there are 

 possibilities of another sense for the ultra-violet actinic 

 vibrations as different from sight as is the infra-red 

 temperature-sense. Moreover, the temperature-sense for 

 us has no scale; there is nothing corresponding to pitch 

 in sound or colour in sight. It may not be so with lower 

 organisms. Insects, for example, may be sensitive to tones 

 of heat. The bee may enjoy a symphony of solar radiance. 



I am not saying that it is so; I am merely suggesting 

 possibilities which we have not sufficient knowledge to 

 authoritatively deny. We have no right to impose the 

 limits of human sensation on the entire organic world. 

 Insects may have " permanent possibilities of sensation " 

 denied to us. 



Even within our limits there may be, as we have 

 already seen, great and inconceivable differences. We saw 



* Professor Langley finds that the maximum effect with a radiating 

 source at 170° C. is at about 5-0 thousandths of a millimetre wave-length. 

 „ 100° C. „ „ 7-5 

 0°C. „ „ 110 

 We are sensitive to radiations from a body at 100° C. But when the 

 temperature falls below the normal temperature of the body we are not 

 sensitive to heat-vibrations, but to loss of heat from the surface exposed. The 

 limit of sensibility to heat-vibrations, therefore, probably lies between 7-5 and 



II thousandths of a millimetre. I have taken about 9-25 as the limit. 



