134 
ELEMENTARY CHEMICAL MICROSCOPY 
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. 67, 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 w; 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 I 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 time the mirror m is turned aside so that no light 
enters 0 . 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- 
