110 ELEMENTARY CHEMICAL MICROSCOPY 



sary to remove the tube A carrying the prism. 1 The slit s is 

 opened to its full width and the microscope focused in the usual 

 manner, the eyepiece having first been itself focused by means 

 of F and set at the proper calibration reference mark c. 



Before the instrument can yield scale readings convertible 

 into wave lengths, it must be calibrated. This will necessitate 

 placing upon its tubes certain reference or indicator marks. 

 The instrument is removed from the microscope tube M, pointed 

 toward the sky and the slit narrowed. The spectrum should 

 appear as a long rectangular band of colored light crossed by 

 many fine black lines at right angles (Fraunhofer's lines) to its 

 length. Should these lines appear inclined, the tube A must 

 be turned slightly until they are made normal to the spectrum 

 length. Having thus carefully adjusted the prism to the proper 

 position with reference to the slit, make the reference marks b 

 upon A and upon r in order to fix this position. Now carefully 

 focus the spectrum by means of F, using the narrowest slit 

 possible until the Fraunhofer lines appear sharpest. This should 

 be done on a bright sunny day. Scratch the mark c to indicate 

 this position. Turn t and tip the mirror n so as to reflect light 

 into the tube and move L until a bright sharp white triangle 

 is seen when looking into the eyepiece. Carefully turn the cap 

 carrying the measuring device until the apex of the bright tri- 

 angle takes a position just a trifle above the center of the spec- 

 trum band. This position is easily ascertained by pushing the 

 comparing prism in place beneath the slit; half the spectrum will 

 now disappear. The most convenient position for the bright 

 spot of light is when the base of the triangle falls just below the 

 dividing line. Make the marks indicated at a so as to fix this 

 position. The instrument is now ready for calibration. It can 

 be taken apart at any time and the parts replaced so as not to 

 alter the values of the scale divisions. After calibration, if, at 

 any future time, wave length measurements are required, the 



1 In other forms of microspectroscopes, as, for example, those manufactured by 

 Zeiss, Leitz and others, the Amici prism is so mounted as to swing upon a hinge 

 above the eye lens. This greatly simplifies adjustments. Unfortunately all of 

 these instruments have measuring devices too crude to be of value to the chemist. 



