Chap, xxvi.] THE MICRO-SPECTROSCOPE. 333 



ways of corroborating the conclusion that blood is 

 present, by the addition of various reagents, and the 

 consequent alteration on the bands, reactions which 

 can be produced by even the one-hundredth of a 

 grain of blood. 



Among other applications the rnicro-spectroscope 

 can be used for the detection of blood in urine. 



Bile gives no spectrum if fresh ; but various com- 

 pounds produced in bile by decompositions and oxyda- 

 tion processes, e.g. by nitric acid, give spectra, by 

 which, therefore, indirect but conclusive evidence of 

 the presence of bile in a fluid can be obtained. 



For demonstrating to a large number of persons 

 at one time the bands of haemoglobin., an oxyhydrogen 

 light, or, preferably, an electric light, is required. The 

 tube of the condensing lens of the lantern is fitted with 

 a cap having a vertical slit with a screw arrangement 

 for making the slit broad or narrow at pleasure. In 

 front of the lantern is placed a convex lens, by means 

 of which an image of the slit is focussed on to a 

 screen several feet in front. A sharp image being 

 secured, a prism is interposed in the path of the beam 

 of light between the lantern and lens, and the 

 prism turned till the best position for disper- 

 sion is secured, indicated by a good spectrum 

 being obtained. The prism frequently employed is 

 a hollow wedge-shaped cell of glass filled with bi- 

 sulphide of carbon, whose refractive index is 1'678, 

 and has, therefore, greater dispersive power than any 

 kind of glass. The rays are, of course, bent out of a 

 straight course and do not fall 011 the screen, but on 

 the walls or objects to the side. To save moving the 

 screen to the side to catch the spectrum, and so losing 

 the proper focus, the lantern, lens, and prism should 

 all be supported on one long board, which rotates on 

 a vertical axis. As soon as a proper position of the 

 prism is secured the board is turned, and the whole 



