256 



PRACTICAL MICROSCOPY. 



FIG. 220. 



so that in many cases Mr. Browning's bright spot micro- 

 meter is decidedly preferable, as it does not preclude the 

 use of either. The lateral piece AA 

 is attached to the upper part of the 

 prism tube, as shown in Fig. 221. At 

 its outer extremity is an oblong box 

 containing a glass plate, upon which 

 is photographed a minute biconvex 

 spot bright on a dark ground. This 



is moved vertically by means of a screw connected with 

 the graduated head M, the spot is illuminated by the small 



plane metallic mirror R, 

 and its image formed by 

 the lens C is capable of 

 being adjusted for focus 

 by the milled collar B. 

 This image is reflected to 

 the eye of the observer 

 from the face of the 

 upper prism, inclined at an 

 angle of 45, and is viewed 

 along with the two spectra 

 between which it appears 

 to pass when the milled 

 head M is turned. In 

 the construction of maps, 

 notice how many revolu- 

 tions of the milled head 

 M will carry the bright 

 _ f spot through the whole 



length of the spectrum, 



which will vary according to the dispersive power of the 

 prisms. 



