USEFUL MICROSCOPE ACCESSORIES 109 



rays entering the system, hence it is usual to term such a prism 

 system, a direct vision prism. The dispersive power of such a 

 system is equivalent to that which would be produced by the 

 prisms of flint glass alone. In the diagram, Fig. 55, the total 

 dispersion indicated is therefore not theoretically correct. 



The measuring device of the Seibert microspectroscope fits 

 above the tube A. It consists of a diaphragm with a very tiny 

 triangular opening I mounted in the sliding plate B and illumi- 

 nated by the mirror n; an image of this opening is projected by 

 the lens / as a tiny bright white triangle upon the inclined surface 

 of the prism R and is then reflected to the eye at i. The knob L 

 serves to slide the lens / and thus focus the image of the triangular 

 opening. The plate in which the diaphragm is mounted can be 

 displaced vertically by means of a micrometer screw; the amount 

 of displacement is indicated upon the scale S and by the gradua- 

 tions upon the drum g; one complete rotation of the drum (100 

 divisions) is equivalent to one division of the scale S. 



To facilitate the illumination of the diaphragm opening I, the 

 mirror n is attached to a rotating collar t. 



The position of a line in the spectrum is ascertained by bring- 

 ing the triangle image to such a position that the line bisects the 

 vertical angle. The scale and drum divisions are then read and 

 recorded. The equivalent of this reading in wave lengths is 

 obtained from the calibration of the instrument by the method 

 given below. 



Should the object, whose absorption spectrum is to be studied, 

 be so small that its image fails to completely fill the length of the 

 slit, the slit must be shortened until the object completely fills it 

 and there will be no light reaching the eye which does not first 

 pass through the object. This is accomplished by pushing the 

 comparing prism into place, thus cutting the spectrum in half. 

 At the same tune the mirror m is turned aside so that no light 

 enters O. Should the image of the object still fail to fill the 

 length of the slit, the sliding diaphragm d is moved toward the 

 center by turning the head D, until the slit length is reduced to 

 the proper dimensions. 



In order to center the object, examine and focus it, it is neces- 



