LIGHT AND ITS ARTIFICIAL PRODUCTION. 287 



proper consideration to these secondary actions of tlie different sources 

 of light it is surprising that, in the choice between the two kinds, the 

 consideration of cheapness plays such an important role. In addition, 

 the danger of an explosion is entirely excluded in an electric light, and, 

 moreover, the process of turning it on is an exceedingly simple one. 



Having considered the nature and performance of the sources of 

 illumination in common use, we will proceed to the third part of our 

 subject, in which we will familiarize ourselves with the physical reasons 

 why the intensity and color of dift'erent lights is so different, and in 

 which we will discuss the factors on which the luminosity of a source 

 of light depends. 



Part III. 



THE PHYSICAL LAWS OF LUMINOSITY. 



The radiation of light and heat. — The sensation of light is purely sub- 

 jective, as has already been pointed out. A pressure on tbe eye is 

 sufficient to produce it, but ordinarily it is produced by ether waves 

 penetrating to the retina and there producing the sensation. 



A source of light — the sun, for example — excites the optic nerve. 

 The same ray of sunlight falling on our skin and evoking the sensation 

 of warmth would produce in the eye the sensation of light and would 

 decompose the sensitive silver salts of a photographic plate. We 

 therefore speak of "heat rays," " light rays," and " actinic rays" corre- 

 sponding to these three different effects, although the three kinds of 

 rays correspond to motions of the ether, differing principally in their 

 periods of oscillation. Every luminous solid emits ether waves of all 

 possible wave lengths, of which only tbose of a length between four 

 ten-thousandths and eight ten-thousandths millimeter can affect the 

 optic nerve. The existence of the other waves can not be demon- 

 strated by the eye, but can be determined by sensitive physical appa- 

 ratus. [Experiment.] A spectrum is produced on the distant screen 

 by the light emitted by zirconium heated in the oxyhydrogen blow- 

 pipe. The spectrum is similar to the rainbow, produced by the refrac- 

 tion of sun rays in rain drops. Every color of the spectrum corresponds 

 to ether waves of a definite wave length, the wave length decreasing 

 continuously from the red to the blue. This visible spectrum, however, 

 embraces only a small portion of the waves emitted by the zirconium 

 light. Both to the left of the red and to the right of the blue there 

 are ether waves the existence of which can be demonstrated. The 

 former, in the infrared, are called heat rays, for their existence can be 

 shown by sensitive thermometers, such as the radiometer, the tliermo 

 pile, the bolometer, etc., while the latter, the ultraviolet rays, are 

 called chemical rays, on account of their photographic activity. They 

 all agree, however, in that they carry with them a definite quantity of 

 energy which is transformed into heat when intercepted by a ther- 

 mometer. In this respect all the rays emitted by a luminous body are 

 "heat rays," although the energy corresponding to the violet and the 



