WITH THE MICROSCOPE. 221 



Having said so much with reference to the instrument, it will be 

 well to describe the manner of preparing and viewing the objects ; 

 and this will be better understood if we first consider some of the 

 general principles involved in this branch of research. 



319. Of Examining Objects in the Spectrum Microscope. 

 Having properly arranged the instrument, if nothing intervenes to 

 interfere with the white light employed for illumination, of course a 

 simple and continuous spectrum is seen, with all the colours from the 

 extreme red to the extreme blues and lavender ; and, if a perfectly 

 colourless and transparent substance be placed in front of the 

 object-glass, no effect whatever is produced, and thus so to speak, 

 all colourless bodies give the same spectrum and cannot be distin- 

 guished by means of their spectra. Coloured bodies are, however, 

 those which are, as it were, black and opaque for certain rays, not 

 allowing them to pass forward as light, but probably transforming them 

 into heat or some other kind of force ; and on placing such a sub- 

 stance in front of the instrument its presence is shown, not by the 

 light which is still transmitted, but by that which it ads off. It is, 

 therefore, more simple and accurate to take into consideration the 

 characters of the absorbed than of the transmitted rays, and in fact, the 

 whole subject of qualitative analysis by means of the spectrum micro- 

 scope, is founded on the relation between different substances and 

 particular rays of the spectrum which they absorb, or so alter that 

 they no longer pass forward as light. Unfortunately, it is not ever)' 

 substance whic'h gives such a spectrum that its true nature can be 

 recognised at once, but many are of such a character that they could 

 not be confounded with any other yet known. These are those which 

 absorb the light in narrow and well-defined portions of the spectrum, 

 so as to give spectra with one or more definite black bands. The 

 number, positien, width, and intensity of these absorption bands are 

 the most important data on which to form an opinion respecting the 

 nature of the substance under examination. It must not be thought 

 that these bands bear any relation to the elementary constituents of 

 the substance they are merely related to it as a definite compound 

 in a particular physical condition, and may vary according to its 

 state. For example, they often vary for the same substance, when 

 solid or in solution ; and even according to the nature of the solvent, 

 besides being greatly modified by the presence of free acids or alkalies. 



Fig. 321, pi. L, gives a few spectra, to illustrate the general sub- 

 ject. They are all of red or pink colours. 



A is an indefinite spectrum, yielded by very many different 

 substances, having a general absorption over the green, with no 

 narrow absorption band. 



