36 Experiments and Observations on the Solar Rays. 
radiate caloric. There is reason to believe that white and color- 
ed opaque bodies continue to discharge visible rays some time af- 
ter they are deprived of all accessions of light from any external 
source. When there is snow on the ground, its whiteness is per- 
ceptible, let the night be ever so dark and cloudy. This opinion 
is also strengthened by the circumstance of some of the lower 
animals, owing to a more delicate structure of the eye, being able 
readily to distinguish objects in what would be total darkness to 
man. ‘The pupil admits of greater dilation in the cat, in the 
owl, in night birds, and in general in all animals that see in the 
dark. The retina likewise appears more sensitive in those ani- 
mals; several of them appear incommoded by the light of day, 
and never pursue their prey but in the most obscure darkness.’”* 
That light, after being absorbed by opaque bodies, can exist 
within them in a free state, and then escape from their pores by 
a self-repellent power of its particles, is admitted by Sir Davin 
Brewster. In the latter part of the eighth chapter of his Life of 
Newton, in which he advances a new theory of inflection, I = 
the following passage. 
“That the particles of light, like those of heat, are endewel 
with a repulsive force which prevents them from: accumulating 
when in a state of condensation, or when they are detained by 
the absorptive action of opaque bodies, will be readily admitted.” 
It appears to me somewhat remarkable, that this eae idea 
was excluded from his “ New Theory of Colors.” | 
The proper colors of opaque bodies certainly depend tat little 
upon the light which is reflected from their external surfaces. 
Let a small piece of black marble which has a flat surface well 
polished, be held in a particular manner between the eye and a 
sheet of white paper. Every part of the flat surface of the mar- 
ble will reflect to the eye a part of the image of the paper, so 
that the whole of the surface will appear white. Instead of the 
sheet of paper, substitute a large piece of red cloth, and the sur- 
face of the marble will appear red; if yellow cloth be substitu- 
ted for the red cloth, the surface of the marble will appear yel= 
low, and so on. Now this shows that the native blackness of the 
marble is owing to no deficiency of reflecting power. But fur- 
. coal reflects from its external surface one twenty fifth of the 
> ean algal and a white body, such as chalk, reflects about 
* Richerand’s Physiology, chap. vit, p. 215. 
