ACCORDING TO 1 H£ UNDULATORY THEORY. 487 



So long as we take 2 b less than from three to four times the length 

 of the wave of red light, we obtain only one place of the spectrum, or 

 both its ends absorbed ; but if we increase b still more, that is if we 

 advance fig. 2 still more fonvards, we perceive that more maxima and 

 minima appear in the spectrum, the more indeed the greater 2 6 is 

 taken. If we suppose 2 i = 0'004 of an English inch, we obtain about 

 tlie same number of absorptions as by iodic gas. 



I have endeavoured to produce artificially those kinds of retardations 

 which the phaenomena of absorption presuppose, and have been so for- 

 tunate as to produce in a very simple manner any kind of the phaeno- 

 mena of absorption I chose. The simplest, and, as I have found, the 

 easiest, manner of performing this experiment successfully is the 

 following ; Bend a piece of a thin plate of mica so that it forms the 

 surface of a perpendicular cylinder ; then place at some distance a 

 lighted candle at the same elevation. The flame which is reflected 

 towards the eye from the cylindrical surface must now appear as a slen- 

 der vertical line. This light is reflected partly from the front surface 

 of the mica, partly, once or more than once, from its hind surface ; 

 the retardation of the last part is to that of the first in proportion to a 

 distance whose magnitude depends on the thickness of the mica. If 

 the thickness of the mica is at all considerable in proportion to the 

 length of tlie wave of light, that is about 0*001 inch and more, the re- 

 flected light appears quite uncoloured. But if we divide this light into 

 colours by means of a prism, and observe the spectrum through a tele- 

 scope, it appears, from the most external red to the most external violet, 

 fillej:l with stripes, which are quite black, and more numerous the 

 tiiicker the plate of mica is. After having shown how we can explain 

 a great number of phffiuomena of absorption by the supposition of a 

 single retarding cause, I will endea\our to show how we may explain 

 all the rest by a further supposition of many other similar retarding 

 causes. If we suppose light of the intensity a, which has been sub- 

 jected to tlie action of a retarding medium, and through that brought 

 down to the intensity 



V 



1 — 2 r'2 cos 2 TT ?:^ + r* 



and subjected anew to a fresh retardation, wliich of itself would have 

 caused the intensity 



a (1 — r'y 



V 



9/i' 



1 -2r'^cos2 7r_ +r'* 

 X 



If is evident that the result A' of bjth retardations must be 



