SIR G. STOKES, BART., ON THE PERCEPTION OF COLOUR. 263 



attention was recently called to some curious experiments by 

 Blix* and Goldschneider.f 



In investigating; the seat of the perception of heat or cold, 

 they used a Bmall body ending in a surface of very small 

 area slightly warmer or colder (suppose in the first instance 

 warmer) than the skin, and applied it in succession tn all 

 points in a selected area of skin, chosen, say, at the back of 

 the hand. It was found that certain points Avere sensitive to 

 heat, while elsewhere the skin was indifferent. The sensitive 

 points ("heat points" as they may be called), when founds 

 were marked with a particular colour. A similar experiment 

 was then tried with a surface colder than the skin, and a 

 group of sensitive points, " cold points," was thus deter- 

 mined, and marked with a different colour. The marking 

 allowed of the experiments being repeated, it might be, on a 

 different day, so as to make sure of the result. It is particu- 

 larly to be noted that we have not got a group of points 

 sensitive to a change of temperature but two distinct groups, 

 one sensitive to heat but not to cold, the other sensitive to 

 cold but not to heat. Here and there a heat point and a cold 

 point might coincide, or rather lie so close as not to be 

 distinguishable in position. 



Similar experiments were tried as to finding out points' 

 which were sensitive to pressure, just the gentlest possible 

 touch of a very small body so as not to cover a large area» 

 and a third group of sensitive points, quite distinct from the 

 two former groups, was thus obtained. It appears from these 

 experiments that a different set of nerve fibres is concerned 

 in communicating to the sensorium the sensation of heat 

 from that concerned in communicating a sensation of cold,, 

 and a different set again of nerve fibres concerned in the 

 sensation of touch. These nerve fibres seem to be very 

 numerous, and to lie pretty close together in some parts of 

 the body, and in other parts more widely apart. 'I'hat, 

 therefore, leads us to regard as not incredible the supposition 

 that in the group of fine nerve threads coming from one of 

 the cones or rods of the retina, there might be nerve threads 

 of different kinds that are capable respectively of producing* 

 different sensations as to colour. No doubt the microscope 



* Zeitschrift fiir Biologie, vol. xx (1884), p. 141, and vol. xxi (1885), 

 p. 145. 



t Archiv fur Anatomie und Physiologie Physiologische Ahtheilung. 

 SuDplement-Band, 1885. p. 1. 



u 2 



