264 THE POPULAR SCIENCE MONTHLY. 



attributes of the ray of that particular wave-length, and are not evi- 

 dences of separate identities. I can no more split that ray up into 

 five or six different rays, each having different properties, than I can 

 split up the element iron, for instance, into other elements, one pos- 

 sessing the specific gravity of iron, another its magnetic properties, 

 a third its chemical properties, a fourth its conducting power for 

 heat, and so on. A ray of light of a definite refrangibility is one and 

 indivisible, just as an element is, and these different properties of the 

 ray are mere functions of that refrangibility, and inseparable from it. 

 Therefore when I tell you that a ray in the ultra-red pushes the in- 

 strument with a force of one hundred, and a ray in the most luminous 

 part has a dynamic value of about half that, it must be understood 

 that the latter action is not due to heat-rays which accompany the 

 luminous rays, but that the action is one purely due to the wave- 

 length and the refrangibility of the ray employed. You now under- 

 stand why it is that I cannot give a definite answer to the question, 

 " Is it heat or is it light that produces these movements ? " There is 

 no physical difference between heat and light ; so, to avoid confusion, 

 I call the total bundle of rays which come from a candle or the sun, 

 radiation. 



I found, by throwing the pure rays of the spectrum one after the 

 other upon this apparatus, that I could obtain a very definite answer 

 to my first question, " What are the actual rays which cause this 

 action ? " 



The apparatus was fitted up in a room specially devoted to it, and 

 was protected on all sides, except where the rays of light had to pass, 

 with cotton-wool and large bottles of water. A heliostat reflected a 

 beam of sunlight in a constant direction, and it was received on an 

 appropriate arrangement of slit, lenses, prisms, etc., for projecting a 

 pure spectrum. Results were obtained in the months of July, August, 

 and September ; and they are given in the figure (Fig. 5) graphically 

 as a curve, the maximum being in the ultra-red and the minimum in 

 the ultra-violet. Taking the maximum at 100, the following are the 

 mechanical values of the different colors of the spectrum : 



Ultra-red 100 



Extreme red 85 



Red 73 



Orange 66 



Yellow 57 



Green 41 



Blue 22 



Indigo 8i 



Violet 6 



Ultra-violet 5 



A comparison of these figures is a sufficient proof that the mechanical 

 action of radiation is as much a function of the luminous rays as it is 

 of the dark heat-rays. 



