EXPLORING THE ATOM 



the color red represents relatively long light waves, 

 whereas violet represents short waves. It now be- 

 came obvious that the eye detects only a small part 

 of the series of ethereal vibrations, and that all radi- 

 ant energy given off by a luminous body includes a 

 long series of waves on either side of the visible 

 spectrum, each series having its peculiar and char- 

 acteristic effects. 



THE PRESSURE OF LIGHT 



This dissection of the ray of radiant energy was 

 made, as we have seen, with the aid of the spectro- 

 scope. It is obvious, however, that an ordinary beam 

 of light, before it is split up by a prism, must contain 

 the entire series of waves of energy heat waves, 

 light waves, and ultra-violet waves blended and 

 intermingled. We have just seen that different 

 portions of those waves may be tested by the ther- 

 mometer (or by its more delicate counterpart the 

 bolometer), by the eye with or without the aid of 

 the spectroscope, and by the photographic plate. 



But there is a joint effect of the waves of radiant 

 energy which may be interpreted in terms neither 

 of heat, light, nor photographic effect, but in terms 

 of physical pressure. 



The exquisitely delicate instrument which meas- 

 ures this effect is called the radiometer. It was de- 

 vised by two American physicists, Professors E. F. 

 Nichols and G. F. Hull, and it is in effect a more 

 delicate modification of an apparatus first made by 

 the English physicist Professor Crookes. The little 

 instrument demonstrates that the waves in the ether 



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