34 PHYSIOLOGY. 



either strong or weak. If the force of impression, which is at 

 40, is increased to 50, it shall appear strong ; but if one at 60 

 is succeeded by one at 55, this will appear weak to us. I give 

 the following experiment in illustration : 



" I take it for granted that the sensations of heat and cold arise 

 merely from different degrees of heat. If I take a vessel with wa- 

 ter, heated to 40, another heated'to 60, and a third heated to 

 50 ; and if I put my two hands into the water at 40 and at 

 60, I shall have no comparison in feeling either remarkably 

 cold or hot ; but if I take the hand immersed into the water at 

 60, and put it into that at 50, it will appear cold ; but the 

 other hand, removed into the same water, will feel hot. Thus, 

 heat and cold are felt accordingly as the single hand was accus- 

 tomed to a greater or less degree of heat. 



" In the same manner, M. du Fay, of the French academy, 

 being employed in ascertaining the property of the various pre- 

 cious stones of shining in the dark, after having been exposed to 

 light, found that the most certain way of examining them, was 

 to have them placed in his chamber after having been in the 

 light, while his chamber was perfectly darkened. Upon awak- 

 ing from his sleep, he, in this way, found many luminous which 

 would not appear so in the middle of the day. When he dis- 

 posed his stones, he did so in a position which required the 

 admission of some light ; and he perceived that his eyes were not 

 sufficiently sensible after being exposed to this light. He tied 

 up one eye, therefore; and, having arranged the stones, he dark- 

 ened the chamber. He then perceived no light with the eye 

 which had been open ; but with the other, which had been 

 closed, he perceived the lights of the stones. 



" I shall add another illustration from my own observation. 

 If I sit in a chamber enlightened by one window, and am in- 

 tent on the image of that window, upon shutting my eye the 

 image still remains as before ; but now the window partitions re- 

 main enlightened and the panes of glass dark. I know that 

 this succeeds with some other persons ; and indeed the remaining 

 of the image is owing to this that the light still comes through 

 my eyelids ; but now the parts which had been in the strong 

 light have their sensibility diminished, and those which had 

 been exposed to the image of the partitions have their sensibil- 



