13° ELEMENTS OF APPLIED MICROSCOPY. 



serum test for typhoid fever. If human blood- serum be 

 injected into the lymph spaces of a rabbit, there appears 

 in the blood of the rabbit a substance which precipitates 

 certain albuminoids in the blood of man; and this reac- 

 tion serves to distinguish human blood from that of the 

 domestic animals with certainty. Only with the cor- 

 puscles of apes and monkeys does the serum of the immu- 

 nized rabbit react in a similar manner. This test, which 

 ordinarily bears the name of Bordet, who first suggested 

 it, is not applied under the microscope, but in small tubes 

 in which the precipitation is detected macroscopically. 



3. The Microspectroscope and Its Use. — ^In testing 

 blood-stains, and in many other fields of microbiology 

 and microchemistry, the spectroscope may furnish aid. 

 As ordinarily used, the instrument consists of a tube 

 fitting on the eyepiece of a microscope and containing, 

 as shown in Fig. 49, a triangular prism of flint glass 

 which disperses white light into its constituent colors 

 and two prisms of crown glass set in the opposite direc- 

 tion, which serve to counteract the refraction produced 

 by the first prism. The principle is the same as that 

 of the achromatic objective, but whereas in the latter 

 the angles are arranged to produce a net refraction with 

 neutralized dispersion, the prisms of the spectroscope 

 yield a net dispersion with practically no refraction. 



The special type of eyepiece used with the micro- 

 spectroscope contains a diaphragm which only allows a 

 longitudinal slit of light to pass, and the width of the 

 slit is regulated by a set screw, being cut down for use 

 to half a millimeter or less. When the detachable prism 



