132 ELBMENTS OF APPLIED MICROSCOPY. 



The sensation of light is produced by the ether vibra- 



271 T-KK 



tions, whose length lies between and 



10,000,000 10,000,000 



of an inch. When any solid body is gradually heated 

 it becomes luminous, first emitting the longer red rays 

 at a rate of 458 million of millions per second and later 

 the rays of other colors as well, culminating with the vio- 

 let rays, whose rate is 727 million of millions. At this 

 point the body has reached a white heat, since rays of 

 all colors are given off. If now the white light from such 

 an incandescent body be passed through the spectroscope, 

 it is broken up, through the different refrangibihty of 

 the rays of different amplitude, into a continuous spectrum 

 in which all the rainbow colors appear merging into each 

 ether. An incandescent gas, on the other hand, pro- 

 duces a line spectrum, most of the field being dark, with 

 here and there narrow bright lines whose number and 

 position are characteristic of the particular substance. 



The color of objects is due to their property of trans- 

 mitting or reflecting rays of a certain amplitude. If 

 white light be passed through certain solids, liquids, or 

 gases below their point of incandescence, and then 

 through the spectroscope, the presence of black bands 

 crossing the spectrum shows that light of certain definite 

 wave-lengths has been removed. A gas cuts out the 

 same light-waves which it itself produces at a higher 

 temperature. 



The spectrum produced by sunlight is a continuous 

 one, like that produced by other incandescent solids, 

 crossed here and there by dark bands, the Frauenhofer 



