136 



ELEMENTARY CHEMICAL MICROSCOPY 



instrument is first set so that all the reference marks take the 

 same positions as when the spectroscope was first adjusted. 



Measurements of line or band positions are made by bringing 

 the bright white triangle to such a position that the line or the 

 edge of the band bisects the acute angle of the triangle. The 

 scale S and drum g are then read and recorded. S reads from o 

 to 70, g in hundredths of S. For example, in the instrument 

 illustrated: Fraunhofer c = 0.42, D = 1.41, G = 7.11, etc. 



In calibrating by means of the Fraunhofer lines direct sunlight 

 should be thrown into the instrument by means of the microscope 

 mirror. For bright lines, hold the instrument clamped securely 

 in place on a suitable clamp stand and direct it toward a Bunsen 

 burner flame into which the metallic salts are to be introduced. 

 The following lines will be found convenient for the calibration : 



When only approximate results in terms of wave lengths are 

 needed, a very convenient device consists in plotting the curve 

 for the spectroscope upon coordinate paper, using wave lengths 

 as ordinates and scale divisions as abscissas. Such a calibration 

 curve is shown in Fig. 68, the black dots indicating the measure- 

 ments actually made. 



For the study of the absorption bands of liquids under the 

 microspectroscope, the most convenient cells will be found to be 

 tubes of different size bores and lengths whose ends are ground 

 true at right angles to their axes. A piece of compact cork pro- 

 vided with a central orifice is cemented to a glass object slide by 



