I876.J 
Mechanical Action of Light. 
235 
other, and you see how perfectly it obeys the force of the 
candle. I think the movement is almost better seen with- 
out the screen than with it. The fog, which has been so 
great a detriment to every one else, is rather in my favour, 
for it shows the luminous index like a solid bar of light 
swaying to and fro across the room. The warmth of my 
finger, or the radiation from a candle, is therefore seen to 
drive the pith disc away. Here is a lump of ice, and on 
bringing it near one of the discs the luminous index promptly 
shows a movement of apparent attraction. 
With this apparatus I have tried many experiments, and 
amongst others I endeavoured to answer the question “ Is 
it light, or is it heat, that produces the movement ?” — 
for that is a question that is asked me by almost everyone ; 
and a good many appear to think that if the motion can be 
explained by an action of heat, all the novelty and the im- 
portance of the discovery vanish. Now this question of light 
or heat is one I cannot answer, and I think that when I have 
explained the reason you will agree with me that it is un- 
answerable. There is no physical difference between light 
and heat. Here is a diagram of the visible speCtrum (Fig. 5). 
The speCtrum, as scientific men understand it, extends from 
an indefinite distance beyond the red to an indefinite distance 
beyond the violet. We do not know how far it would extend 
one way or the other if no absorbing media were present ; 
but, by what we may call a physiological accident, the human 
eye is sensitive to a portion of the speCtrum situated between 
the line A in the red to about the line H in the violet. But this 
is not a physical difference between the luminous and non- 
luminous parts of the speCtrum ; it is only a physiological 
difference. Now, the part at the red end of the speCtrum 
possesses, in the greatest degree, the property of causing 
