64 ELEMENTARY CHEMICAL MICROSCOPY 



phragm D 12 to 15 from Ci and the arc lamp so that its carbons 

 are about 8 centimeters from D. Turn on the current adjusting 

 the rheostat so as to employ a current consumption of approxi- 

 mately 10 amperes and see that the + carbon is the horizontal 

 one. Later when in the prosecution of studies raise the current 

 to one of 15 or even 20 amperes. Move the lens Ci backwards 

 and forwards, at the same time holding a piece of dull black 

 glass, dull black paper, or a piece of ground glass in front of the 

 slit, until a position is obtained which projects an image of the 

 arc of maximum brightness upon the black screen and of such a 

 size as to completely fill the slit opening. Set the slit so that 

 the micrometer screw G is up as shown in the figure, and adjust 

 the opening to about i millimeter by 1.5 millimeters, its length 

 being horizontal. Now hold a black screen against the end of T 

 and move C% back and forth until a very bright and sharp image 

 of the slit is obtained, adjusting Ci again slightly if necessary. 

 Next hold the black screen so that its surface lies in the plane 

 of the optic axis of the observing microscope and adjust the 

 objective 3 so that a very bright, uniform spot of light a little 

 less than i millimeter in diameter is obtained. Turn the fine 

 adjustment V% until the spot of light falls in the optic axis of M. 

 For the final adjustment of the apparatus the cell may be filled 

 with a liquid which contains colloids yielding brilliant diffraction 

 patterns or with a slightly alkaline solution of fluorescein. The 

 path of the illuminating beam is thus easily seen. Focus upon it, 

 using a low power objective and No. i eyepiece and by means of 

 Vi and 2 adjust the beam so that it passes through the center 

 of the field as a narrow thread of light with its minimum diameter 

 at the center of the field. Replace the material employed for ad- 

 justing by the substance to be studied. The only adjustment 

 which should now be required will be the diameter of the slit; 

 if there appears to be required a marked change in slit diameter 

 it is probable that following this change there may be required 

 slight changes of Vi and Vz. 



If the beginner will proceed as indicated little difficulty will 

 be experienced in adjusting the slit ultramicroscope for use. 

 The most annoying feature is the change in the position of the 



