138 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 



the light ; and then, when taken into a dark place, it 

 shines for a time, the light gradually fading away. But 

 the glow can be re-established by raising the tempera- 

 ture, and an exposure of a single second to the sun is 

 quite enough to commence the process. 



To recognize feeble degrees of luminosity, it is neces- 

 sary for the observer to remain in the dark until the 

 pupil of the eye is quite dilated, and the impression of 

 light to which the retina has been exposed is worn off. 

 Du Fay gives a singular but very serviceable practical 

 process. He recommends the experimenter to keep one 

 eye bound up or closed for the purpose of observing in 

 the dark, and to use the other in conducting his process- 

 es in the light. He remarks the curious fact that the 

 eye which has been shut will not have the delicacy of 

 its indications affected by that which has been exposed 

 to the light. 



In~this manner Du Fay found that of four hundred 

 yellow diamonds all were phosphorescent; but some 

 that were white or rose-colored or blue or green were 

 not. Nor was there any external indication by which 

 it could be told whether any given one of these kinds 

 would shine. He discovered, too, that the glow took 

 place under various - colored media, as stained - glass, 

 water, milk, but not under ink. He also made attempts 

 to compel the gem to preserve its light by enveloping it 

 in opaque media, such as ink, black wax, etc., under the 

 idea that the light could not get out, and concluded that 

 he had partially succeeded, because in some instances the 

 diamonds would shine after being so shut up for six or 

 twelve hours. He verified Boyle's fact on the effects of 

 hot water and heating generally, and carried his tem- 

 peratures to far higher degrees, even above a white heat, 

 finding that the stone had lost none of its qualities, for 

 it would take light again when it was cold on a momen- 



