1?3 THE MICROSCOPE. 



the stage of a microscope, and is mounted in a similar 

 manner. By means of this mirror light may be reflected 

 into the eye-piece, and in this way two spectra may be 

 procured from one lamp. 



For observing the spectra of liquids in cells or tubes 

 of considerable diameter, say not less than j^th of an 

 inch, powers from 2 inch to 1 inch will be the most 

 suitable, and of course low powers only can be used to 

 investigate the spectra of opaque objects ; but when the 

 spectra of very minute objects are to be viewed, powers of 

 from half an inch to one-twentieth, or even higher, may 

 be employed. 



Blood, madder, aniline red, permanganate of potash, in 

 crystals or solution, are convenient substances to begin 

 experiments with. Solutions when made too strong pro- 

 duce dark clouds instead of absorption bands. Professor 

 Church has recently pointed out that zircon, an almost 

 colourless stone, gives well-defined absorption-bands. 



Mr. Sorby says of the correct performance of a spectrum 

 adaptation, " The best tests are, first, that the absorption- 

 bands in blood can be seen when they are very faint ; 

 second, to well divide the bands in permanganate of 

 potash; and, third, to see distinctly the very fine line 

 given in the red by a solution of chloride of cobalt dis- 

 solved in a concentrated cold solution of chloride of calcium : 

 there is a line so fine that it looks like a Fraunhofer'a 

 line. An instrument that shows all these well is all that 

 can be desired. 



" The objects most easily obtained, and which furnish 

 as 'with the greatest variety of spectra, are coloured 

 crystals, coloured solutions, and coloured glasses. The 

 spectrum microscope enables us to examine the spectra of 

 very minute crystals, of very small quantities of material 

 in solution, and of small blow-pipe beads. As previously 

 named, the thickness of the object makes a very great 

 difference in the spectrum. For example, an extremely 

 thin crystal of ferricyanide of potassium cuts off all the 

 blue rays, and leaves merely red, orange, yellow, and more 

 or less green ; but on increasing the thickness, the green 

 and yellow disappear ; and when very much thicker, little 

 else but a bright red light is transmitted. In all such 



