44 REPORT ON PHYSICAL OPTICS. 



mutually stops in bodies as often as its rays strike upon their 

 parts."* 



When from the simple fact of absorption we proceed to con- 

 sider its law, as depending on the nature of the light, the difficul- 

 ties increase at every step. The intensity of the transmitted 

 light, considered as a function of its refrangibility, appears to be 

 subject to no law, or to a law so complicated as completely to 

 baffle all attempts to embrace it in an empirical rule. The maxima 

 and minima are often actually numberless ; and the variable does 

 not reach them gradually, but by what seems to be an abrupt 

 violation of the law of continuity. These apparently capricious 

 changes were observed long since by Dr. Young, in the light 

 transmitted through the common smalt-blue glass. Sir David 

 Brewster has recently directed his attention to the same subject, 

 and examined a great number of coloured bodies with reference 

 to their absorptive properties. He has found, in particular, that 

 a very remarkable definite action is exercised upon the rays of the 

 spectrum by the green liquids obtained by extracting the colour- 

 ing-matter of the leaves of plants in alcohol ; and this action does 

 not cease altogether even when the liquid has become perfectly 

 colourless.f But the absorbing properties of .nitrous-acid gas, 

 observed by the same author, are by far the most remarkable ever 

 noticed. "When the light transmitted through this gas is analyzed 

 by a prism, it is found that about 2000 portions of the beam are 

 stopped, and 2000 dark spaces, or abrupt deficiencies of light, 

 appear in the spectrum. These increase in number and magni- 

 tude with the temperature of the gas, until, by a sufficient eleva- 

 tion of temperature, this rare body becomes perfectly opaque, and 

 refuses to transmit a single ray of the brightest sunshine. J Pro- 

 fessor Miller and Professor Daniell have found some analogous 

 properties in other gases. In the spectrum produced by the 

 light transmitted through the vapours of bromine and iodine, 

 more than one hundred dark lines are visible, disposed at equal 

 distances.! 



To account for the selection of certain classes of rays by 



* Optics. Query 30. 



t " On the Colours of Natural Bodies." Edln. Trans., vol xii 



" On the Lines of the Solar Spectrum," & K .-Edin. Trans., vol. xii. 

 t French translation of Herschel's Essay on Light, Supplement, p. 455. 



