ME. TV. CEOOKES ON EEPULSION EESULTING- EEOM EADIATION. 
361 
happens to be capable of affecting the retina of the human eye. There is no real 
distinction between heat and light ; all we can take account of is difference of wave- 
length ; and all we can see in the spectrum is one continuous series of vibrations, longer 
at the red end than at the violet end, but extending in an unbroken series for an unknown 
distance on each side. I say unknown, for it is probable that the whole spectrum, as 
we know it, is limited by the imperfect transparency of the atmosphere, or of the refracting 
medium, for the extreme ultra-red and ultra-violet rays. 
Take a ray of the spectrum of a definite wave-length (the line B for instance), and 
allow it to fall on a thermometer; the mercury rises, showing the action of heat ; 
concentrate it on the hand by a lens, it raises a blister accompanied with pain ; let 
it fall on a bismuth and antimony couple, the galvanometer is deflected ; and this action 
we also call one of heat. Let the ray fall on the eye, and it produces the sensation of 
light and colour. Let it fall on a collodion plate prepared in a particular manner, and 
it gives a permanent image, showing that it can cause chemical action. Lastly, throw 
the ray on a portion of matter free to move in a vacuum, and it makes itself evident as 
motion. Now these actions of heat, light , colour , chemical action , and motion are 
inseparable attributes of the ray of that particular wave-length ; and to consider that 
there can be a splitting-up of this ray into two or more rays of the same refrangibility, 
one having the property of light, the other of heat, &c., is to my mind an absurdity. 
The longer waves of the spectrum are those most able to produce heating-effects, the 
shorter waves best cause chemical action, and the intermediate waves easiest excite the 
sensation of vision ; but although the maxima of these actions are at different parts of 
the spectrum, each effect can be detected at any part. 
In a similar way the production of motion has its maximum in the waves situated at 
the ultra-red part of the spectrum, whilst it is capable of being rendered evident in all 
parts. This at first sight would favour the supposition that the action was due to the 
heating power of the waves. 
193. How far this is really the case may be seen by the following Table, in which I 
have reduced the maximum to 100, and given the motion-producing value of the different 
colours of the spectrum, reduced in the same proportion : — 
Ultra-red 100 
Extreme red 85 
Bed 73 
Orange 66 
Yellow 57 
Green 41 
Blue 22 
Indigo 8-| 
Violet 6 
Ultra-violet 5 
