VI.] 



INSTRUMENTS EMPLOYED. 



71 



reflected by the last surface of the prism up the observing 

 telescope, so that it is seen as a bright scale on the background 

 formed by the spectrum under observation. 



In another method of measurement, the telescope is attached 

 to a movable arm, which can be directed to any part of the 

 spectrum, and the outer edge of the circular stand along which 

 the telescope moves is graduated with more or less minuteness, 

 according to the precision in the position of the lines required 



FIG. 29. Steinheil's form of four-prism spectroscope. A, collimator ; 

 B, observing telescope. 



in the research. In this method the line to be measured is 

 brought into the centre of the field of view by the aid of cross 

 wires in the eyepiece of the observing telescope, and the posi- 

 tion of a pointer carried by the telescope is read off on the 

 graduated arc. 



There are many variations of these methods of measurement, 

 but a description of them belongs rather to a special book on 

 the spectroscope than to the present volume. 



