194 



MESSRS. A. SCHUSTER AND G. HEMSALECH ON 



spectrum should fall near the top of the disc vertically above the axle. I represents 

 the indicator which shows the speed of the motor. 



The adjustments which it is necessary to carry out with accuracy consist in the 

 focussing of the spark image on the slit, the proper centering of the apparatus, and 

 the focussing of the camera lens. In addition to this, it was found convenient to 

 place the lens A as nearly as possible half-way between the spark and slit, so that the 

 spark image on the slit should be of the same magnitude as the spark. This rendered 

 the interpretation of the photographs more easy, as the linear dimensions measured 

 in the direction of the spark discharge were reduced in that case by the optical 

 arrangement in the fixed ratio of the focal lengths of the camera and collimator lenses, 

 viz., '86. One millimetre in length of the slit images therefore always corresponded 

 to 1'16 millim. in length of the spark path. 



Fig. 8. 



The adjustment of collimator and lens A is most easily carried out if the prism C and 

 lens D are removed, so that a clear view can be obtained through the collimator 

 towards the spark box. The collimator was adjusted in the usual way for infinite rays. 

 As it was separately supported on a tripod stand, it could be accurately levelled, 

 and the centre of the slit placed, by means of a cathetometer, accurately at the 

 same height as the centre of the spark gap. The focal length of the lens A being 

 known, the distance between the spark and slit was adjusted by measurement to be equal 

 to a little more than four times that fooal length, and the lens A was placed at the 

 right height, and exactly half-way between the slit and spark gap. Sparks from 

 an induction coil were now allowed to pass between the electrodes, the slit being opened 

 wide, and a telescope adjusted for infinite rays placed in front of the collimator, so 

 that the observer obtained a clear view of the slit. If the adjustment is good, the 



