chap, xxvi.] THE MICRO-SPECTROSCOPE. 331 



already mentioned, being moved so that the slit is nar- 

 rowed till a sharply defined spectrum is obtained. For 

 the spectrum of blood a dilute solution of blood is 

 placed in a small cell on the stage of the microscope, 

 and the light from a mirror transmitted through it. 

 The cell recommended by Sorby is made by taking a 

 piece of barometer tubing half an inch long and one- 

 eighth of an inch in internal diameter. It is cemented 

 vertically on a piece of plate-glass by purified gutta- 

 percha. Either a low or a high power lens may be 

 used, though with high powers the illumination is too 

 weak for colours beyond the green, unless a condenser 

 be used underneath the stage. With such an arrange- 

 ment as has been described, only the spectrum, as 

 modified by the substance on the stage, is observed. 

 There remains to be noted a device for obtaining an ordi- 

 nary spectrum for comparison. In the wall of the wide 

 part of the tube (Fig. 152) is a small opening K, pro- 

 vided with a slit. In front of this opening, suspended 

 from a projecting arm and movable in all directions, 

 is a small mirror I, which reflects light into the tube 

 through K. K is just below the level of the slit E. 

 A small part of the slit E is covered by a small rect- 

 angular prism c, so placed that the reflected light 

 from the mirror passes straight through the near face 

 of the prism, but undergoes total reflection at the 

 internal surface of the diagonal face. The result of 

 the total reflection is to direct the rays from the 

 mirror straight up the tube of the micro-spectroscope, 

 by the prisms of which they are dispersed and a 

 spectrum produced. Thus, through one part of the 

 slit E rays pass from the microscope mirror up through 

 the fluid on the stage producing the clear characteristic 

 spectrum of the substance, while through the other 

 part of the prism rays proceed from the side mirror, 

 which pass through no absorbing substance, and yield 

 an ordinary spectrum. As seen by the eye the two 



