132 ELEMENTS OF APPLIED MICROSCOPY. 
The sensation of light is produced by the ether vibra- 
271 I 
ee ond aes 
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- 
tions, whose length lies between 
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 refrangibility of 
the rays of different amplitude, into a continuous spectrum 
in which all the rainbow colors appear merging into each 
cther. 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’ 
