Spectroscope Apparatus. 239 



exhibit exquisite gradations of colour, from, red at one end to 

 violet at the other. If we attempt to form a spectrum by 

 allowing a broad mass of light to fall upon a prism, we shall 

 only partially succeed. We shall indeed see rainbow colours, 

 but they will be overlapped and confused. If, however, we 

 permit the light to reach the prism through a narrow slit, and 

 exclude extraneous rays, the confusion will be avoided, and a 

 neat, well-defined ribbon of parti-coloured light will be ob- 

 tained, in which, at certain intervals, dark lines will be observed. 

 These lines, which would appear to a casual observer as insig- 

 nificant as so many threads of a cobweb, are the hieroglyphic 

 letters which the spectroscopist has to decipher, in accordance 

 with the principles explained in the articles to which we have 

 already made reference. To Sir J. Herschel belongs the credit 

 of first pointing out that as the spectra of incandescent vola- 

 tilized substances differed from each other, they might be made 

 use of for purposes of analysis. There are only two modes in 

 which such spectra can differ from each other, or from the solar 

 spectrum which may be taken as a standard. Either they will 

 afford bright lines in the place of dark lines, or dark lines in 

 place of light ones. We have used the term lines, but when 

 the breadth is considerable, spaces or bands is a better 

 appellation. 



When a glass prism acts upon a ray of light so as to give 

 a spectrum, two distinct actions take place. First, the light 

 ray is refracted, or bent out of its course; secondly, it is 

 opened or spread out like a fan. This last action is called 

 dispiersion, and, like refraction, its amount varies with the sub- 

 stance employed in the formation of the prism. 



For the detection of the metals and many other substances, 

 a very minute examination of the spectrum is seldom re- 

 quired, as the spaces or bands by which their presence is 

 indicated are conspicuously shewn by any instrument that 

 will display the chief lines of the solar spectrum. There are, 

 however, many thousand finer lines that can be discerned if a 

 sufficiently powerful and delicate apparatus is employed, and 

 many lines that appear single under ordinary circumstances 

 are found to be multiple when examined with superior means. 



The size and character of the spectroscope should be regu- 

 lated by the work required of it. A very small one, that can 

 be carried in the waistcoat-pocket, will serve to indicate the 

 presence of the most characteristic substances, or to show to 

 the traveller whether the atmosphere — as is often the case of 

 an afternoon and in thunder-storm weather — developes any 

 lines of moderate magnitude that are not regularly seen. A 

 small spectroscope however labours under two obvious limita- 

 tions : Its prism cannot usefully receive much light ; opening 



