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familiarized with the idea of its accumulation, condensation, 
fixation, liberation, &c. that in the end this doctrine appears 
to contain nothing but what is simple and easy of compre- 
liension. 
'If the light which shines on us was in reality a substance 
which luminous bodies sent to us, we might hope to be able 
to discover how and where it existed before it was trans¬ 
mitted to us; but if it is only an impulse (coup) given to 
the eye, by the repercussion of an elastic fluid which touches 
the eye, and at the same time all the bodies in the universe, 
we should lose time either in seeking its source or its che¬ 
mical properties. 
No person I think ever advised us to search for sound in 
fulminating powder; is it more reasonable to seek in the 
substance which is burnt for the light which it emits ? 
The light which stares us in the face only exists in the 
eye. 
The eye is formed in such a manner as only to be sensi¬ 
ble to impressions of a limited force, and to vibrations of a 
certain velocity ; all others are without effect on this organ. 
When a common ball is heated red hot and set in a dark 
place to cool, it ceases to be visible to the human eye as 
soon as it is cooled down to the 900° of Fahrenheit’s ther¬ 
mometer; but can it be believed that the ball experiences 
a sudden and notable change in its state at the instant it be^ 
eomes invisible ? This supposition is unreasonable. 
An inhabitant of another planet, that for example disco¬ 
vered by Herschell, if he had eyes differently constructed 
from ours, and more sensible to a small degree of light, 
looking with the man of this globe at the ball as it was 
cooling, would be greatly astonished to hear him say, sud¬ 
denly, “ the ball has ceased to be luminous.” 
It is possible, and even probable,.that there are animals 
©n our own globe who perceive bodies shine after they cease 
to be luminous to us, and perhaps some that we are never 
able to see. 
