ACCORDING TO THE UNDULATORY THEORY. i39 



sorption proceeds toward the red end of the spectrum, new black stripes 

 arise before them, till at last, at a certain intensity of the colour of the 

 iodic gas, the whole spectrum is absorbed, with the exception of a small 

 piece of the red, Avhich is now entirely filled with black stripes. This 

 beautiful phasnomenon of absorption is explained with the greatest ease 

 and exactness by supposing two different causes of retardation. We 

 need only presuppose that the one I'etardation is about equal to the 

 length of the M'ave of red light, and the other about 1 50 times greater. 

 The part of the curve of the intensity which represents the first re- 

 tardation has the form A B (fig. 3) ; but that part of it which repre- 

 sents the second has the form C D (in the same figure). The results 

 of both may therefore be expressed by a curve of the form of E F. 

 With increasing intensity of the colour of the iodic gas, we must sup- 

 pose that r and r' are increased, and that this increase can of couree 

 have an influence, not on the station of the maxima and minima, but 

 merely on tiie absolute magnitude of the intensity. The greater r and 

 r' become, tlie less must also the intensity become. And as we must 

 imagine that the light, in order to be visible, must have a certain inten- 

 sity, and as we can also express these limits of pei'ception of the light 

 by a line, it is evident that the increase of r and r' must force down 

 the curves of the intensity toward this line. 



If we now place fig. 2 on fig. 3, and conceive the line A B in 

 fig. 2 to represent the limit of perceptibility for the eye, we shall 

 render the effect of the increase in the value of r and r' evident by 

 sliding fig. 2 gradually higher on fig. 3. If fig. 2 lies on the line 

 u b, we see some stripes appear in the blue ; if we move it higher 

 towards a' b', we see that the blue end is absorbed, and the stripes now 

 make their appearance in the green ; if we move it still higher to a" b", 

 we find the whole spectrum absorbed, with the exception of a piece of 

 red, which is now filled with black stripes. This is precisely the pro- 

 cess in the spectrum of the iodic gas. The phaenomena of absorption in 

 bromic gas are explained just in the same manner. In order to ex- 

 plain the spectra arising from the light which has traversed nitric acid 

 gas, or euchlorine, we must suppose several causes of retardation. We 

 must not be astonished at this difference, as the two last gases belong 

 to the compound, while the two first belong to those which we consider 

 as simple. It appears to me quite natural to assume that the ele- 

 mentary constituent parts of a compound body may each of itself cause 

 different retardations; and if we consider nitric acid gas as a compound 

 of nitric acid and nitrogen, instead of considering it as a binary com- 

 pound of azote and oxygen, we then easily conceive how very possible 

 it is that a grtiat nundier of retarding causes may be contained therein, 

 each of which arises in the sanu; manner as in the single gases. 



Without endeavouring to explain the presupposed causes of retarda- 



