56 Mr. James Bice on 



750 billions of vibrations per second, and for extreme red light 

 about half of this number. (Billion is taken to mean 10^^ or 

 one million millions). 



We are able to do more than measui'e the wave-length of a 

 stream of radiation ; we can also measure the amount of energy 

 which is carried per second across any section of its path. To 

 do this the cross-wire of the telescope in the spectrometer is re- 

 placed by an extremely thin strip of some metal coated with a 

 highly absorbent material such as lamp black. The energy of the 

 concentrated stream from the grating is converted into the 

 imiversal currency for measuring energy, viz., heat, in the strij). 

 The strip itself forms part of an extremely sensitive mechanism 

 in which an electric current is generated by the heat thus 

 developed, this current being measurable by a delicate mirror 

 galvanometer. The seiisitiveness of such radiomicrometers " is 

 such that a rise of one millionth of a degree in the temperature 

 of the strip is observable with accuracy. 



The possibility of using such radiomicrometei's is of extreme 

 importance ; for, as is well known, there exist I'adiations which 

 do not affect the retina. Our only direct experience of them is 

 the sensation of glow when we stand before a fire or hold our 

 hand near a hot kettle, etc. Such radiations are poured forth in 

 profusion from all bodies, hot or cold ; only at the extreme zero 

 of temperature, the so-called absolute zero, would any body cease 

 to radiate. This fact generally eludes us because of our lack of 

 direct sensation in many cases, although we occasionally experi- 

 ence one result of it, viz., the chilly sensation near a very cold 

 body, arising from the condition that we are radiating more to 

 that body than it is radiating in return to us, involving a net loss 

 of energy on our part. At all events their existence is undoubced. 

 They can be directed from bodies, luminous or not, to a grating ; 

 the diffracted streams from the grating can be detected by means 

 of a radiomicrometer, and measured both as regards wave-length 

 and intensity. The photographic plate is also an extremely use- 

 ful appliance in radiometric work ; the camera can replace the 



