THKEE DIFFERENT EMANATIONS. 201 



stitutes the branch on the left. The third branch contains 

 a micrometer scale, so called, an image of which is project- 

 ed upon the spectrum, and enables the observer to deter- 

 mine the precise position of any bands which may be 

 brought to view. By observations made with this instru- 

 ment spectral analysis, as it is called the radiation which 

 comes from the sun, as well as that from many other lu- 

 minous sources, is found to be of an exceedingly complex 

 character. The portion of it which constitutes light that 

 is, which has the power of producing in our minds the sen- 

 sation of vision, can be separated into a great number of 

 distinct parts, each of which produces a different sensation 

 in our minds in respect of color; and, besides the light, 

 there are rays of heat, and a third kind still, called chem- 

 ical rays, a considerable portion of both of which can be 

 entirely separated from the rays of light in the spectrum. 

 Fifty years ago, a commonly received theory was that all 

 this radiation consists of streams of solid particles, which 

 were both inconceivably minute, and were projected from 

 their source with inconceivable velocity. The prevailing 

 theory at the present day is that they consist of different 

 modes of vibration or undulation, in an extremely rare and 

 tenuous ether, w r hich is supposed to pervade all space, and 

 even to fill the interstices of solid bodies. The longer and 

 more slow of these vibrations if the words long and slow 

 can be applied at all to movements so amazingly minute 

 and rapid as these are all supposed to be constitute, it is 

 thought, the rays of heat. Those of somewhat greater ve- 

 locity and minuteness form the light, while the most minute 

 and rapid of all are the rays of chemical action. In other 

 words, that the vibrations of the first class that is, those 

 that are comparatively slow, have the power to affect us 

 with the sensation of heat ; those of the second class have 

 the property of producing impressions upon our organs of 

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