250 



PRACTICAL MICROSCOPY. 



scopy to be provided with several copies of the tube, 

 containing prisms yielding different degrees of dispersion. 

 Mr. Browning has made for the author four such tubes, 

 producing a small, medium, and large amount of dispersion, 

 one of which can easily be substituted for the other. 



FIG. 216. 



The student must remember that an increased dis- 

 persion does not define the absorption bands any the 

 better ; on the contrary, fine and delicate lines, easily 

 seen by the use of a prism with moderate dispersion, are 

 often lost when one of large dispersion is substituted. 



To use this instrument, insert it as an eye-piece in the 

 microscope, screw on the object-glass, and place the object, 

 the spectrum of which is required, upon the stage. Illu- 

 minate by means of the sub-stage mirror if the object is 

 transparent, or with mirror, Lieberkuhn, or side-reflector if 

 opaque. Remove the tube A, and open the slit by means 



