42 



that there is only one active agent at one part of the spectrum, 

 for towards the left extremity, or the red end of the spectrum, 

 there is at the same time a heating and illuminating effect. 

 These different rays possess distinctly heating, luminous, and 

 chemical properties ; there is, however, a great difference 

 between the first of these and the two latter, " for the heating 

 effect of a ray may be made the physical measure of the 

 power which this portion of the spectrum possesses." 



When light which has emanated from different sources is 

 decomposed by a prism, the spectra which are obtained may 

 differ in several important respects from each other, but all of 

 them can be conveniently arranged in three general groups. 



The special character which distinguishes spectra of the 

 first order is that the continuity of the colored band is un- 

 broken either by dark or bright lines. We learn from such a 

 spectrum that the light has been emitted by an opaque body, 

 and most probably by matter in a solid or liquid state. A 

 spectrum of this order gives no knowledge of the cbemical 

 nature of the incandescent body from which the light comes. 



Spectra of the second order are very different. These con- 

 sist of colored lines of light, separated from each other. From 

 such a spectrum we learn that the luminous matter from 

 which the light has come is in the state of gas. Every com- 

 pound body that can become luminous in the gaseous state 

 without suffering decomposition is distinguished by a group of 

 lines peculiar to itself. Substances lolien in the state of gas 

 may be distinguished from each other by their spectra, pro- 

 viding the lines characteristic of the different terrestrial 

 substances are known. The discoveries effected by Mr. 

 Huggins have shewn that many of the Nebulse are of gaseous 

 composition, presenting the spectrum of bright lines separated 

 by dark spaces. 



The tliird order consists of the spectra of incandescent 

 solid or liquid bodies, in which the continuity of the colored 

 light is broken by dark lines. These dark spaces are not 

 produced by the source of the light ; they are produced by 

 vapours through which the light has passed on its way, and 

 which have robbed the light by absorption of certain definite 

 colors, or rates of vibration ; such spectra are formed by the 

 Sun and Stars. 



The spectra obtained from the Moon and Planets are 

 unlike those produced by the Stars and Nebulae, these latter 

 not being original sources of light, but shining by reflection 

 only ; their spectra resemble, therefore, a modification of the 

 solar spectrum. 



Circumstances frequently unite in rendering investigations 



