'I'Q'l' BARON VON WUEBE ON THE ABSORPTION OF LIGHT 



gas, but also that which arises through absorption in oxalate of chrome 

 and potash. In these experiments the magnitudes r and r' can be 

 varied up to a certain degree, and consequently the breadth of the 

 black stripes can be changed in proportion to the breadth of the bright 

 ones. For this purpose it is only necessary to let the light fall on the 

 surface of the cylinder, under different angles of incidence. For it is 

 evident that the proportion between the light reflected on the first and 

 that reflected on the second must be greater the smaller the angle of 

 incidence is ; and consequently that the black stripes must at smaller 

 angles of incidence be less broad than at greater. If we wish to produce 

 absorptions with a small difference between the intensities of the maxima 

 and minima, it is only necessary to let the light pass through a plate 

 of mica instead of being reflected on it. In this experiment the propor- 

 tion can be considerably varied by changing the angle of incidence. 



The most complex of all the phaenomena of absorption is undoubt- 

 edly the solar spectrum, with its numerous irregularly placed stronger 

 or fainter black stripes. If we suppose with Herschel that these 

 stripes arise from absorption in the atmospheres of the sun and earth, 

 it becomes easy to explain them according to the principles already 

 laid down. Although I have not yet made any experiments with a 

 view to ascertain whether and in what measure the different pres- 

 sures of the gases act on the position of the arising absorptions, yet 

 I think it highly probable that they have a very considerable influ- 

 ence on it. In such a case it is evident that the light in its passage 

 through both the atmospheres, the density of which varies with the di- 

 stance from their respective bodies, must suffer an indefinite number 

 of unequal retardations, each of which will produce a certain series of 

 maxima and minima. The cause of the number and also of the irre- 

 gular position of the black stripes is consequently easily to be conceived. 

 But the (at least apparently) vast difference which takes place between 

 the intensities of the maxima and minima re(iuires a particular explana- 

 tion, which I will now endeavour to make. We have hitherto considered 

 only two reflecting surfaces ; it is however evident that, according to 

 the hypothesis with which I have set out, we must suppose a series 

 of such surfaces Avhich will be greater in proportion to the greater thick- 

 ness of the absorbing medium. If we call, as before, a the original in- 

 tensity of the light, r the loss in each reflection, and m the number of 

 reflecting sui'faces, we easily perceive that the intensities of the system 

 of waves of light must be as we find them given in the following table 

 (fig. 10), in which A, B,, A^ Bn, A3 B3, &c. represent the reflecting 

 surfaces. 



When the thickness of the absorbing medium is rather considerable, 

 m must be a great number ; and consequently r must be very small, 

 because otherwise no considerable portion of light could traverse all 



