102 



THE MICROSCOPIST. 



the micrometer already described may be used, or Mr. 

 Sorby's apparatus, giving an interference spectrum with 

 twelve divisions, made by two Mcol's prisms, with an 

 intervening plate of quartz of the required thickness. 



The value of this mode of investigation in medical 

 chemistry, and for purposes of diagnosis or jurisprudence, 

 may be seen by the following illustrations:* 



Pettenkofer's Test for Bile (Fig. 55). To a few drops of 

 bile in a porcelain dish, add a drop of solution of cane- 



A a P C 



H TT 



Pettenkofer's Bile-Test. 



sugar, and then concentrated sulphuric acid drop by drop, 

 with agitation. The mixture becomes a purple-red color, 

 and shows a spectrum as in the figure. The color will 

 be destroyed by water and alcohol. 



Tests for Blood. Haematocrystalline, or cruorin, com- 

 posed of an albuminoid substance and hsematin, generally 

 crystallizes in tetrahedra or octahedra. In blood from 



A a B C 



H IT 



Blood. 



the horse and from man only an amorphous deposit is 

 found. The watery solution of this substance properly 

 diluted, shows two remarkable bands of absorption, and 

 obscuration of the blue and violet end of the spectrum 

 (Fig. 56). As the blood of all vertebrates shows the same 



* See Thudichum's Manual of Chemical Physiology. New York, 1872. 



