122 



BELL SYSTEM TECHNICAL JOURNAL 



angles to the principal slit, one being adjustable by a milled screw head. 

 The function of these jaws is to limit the length of the slit. The slit 

 head may be gi\en a quarter turn so that it may be set horizontally or 

 vertically, which is necessiiry in order to calibrate the instrument as 

 explained later. A projection lens next in order toward the micro- 

 scope projects the image of the slit into the image plane of a hori- 

 zontally mounted objective which is mounted on a stand witii cross 



Fig. 22— Ccllb 



■il for ilu- rx.uniiiatiiiH of lliijils willi llie iikr.i-micn 



slides so that the objective which serves as an illuminator nia\' be 

 moved horizontally in two directions, at rigiit angles to each other. 

 The mo\-enient of the cross slides is controlled by screw adjustment 

 but for coarse adjustment in the direction of the illuminating train a 

 sliding sleeve adjustment is made. By this means the illuminating 

 objective can be centered with respect to the obser\ing microscope 

 objective. In the correct position the front lens of the illuminating 

 objective is about 1 mm. from the mount of the obser\ing objecti\e. 

 The Biltz cell has a rectangular cross section which permits of 

 accurately adjusting the cell in position. A thistle funnel at one 

 end is for the reception of the liquids; the other end is provided 

 with a piece of rubber tubing which has a pinchcock to pre\'ent the 

 escape of the fluid. The rectangular section of the cell has two quartz 

 windows, one of which normally faces the illuminating objective and 

 the other the obser\'ing objective. The ceil is attached to the observ- 

 ing objective by means of the clamp mentioned and the cell is focused 

 in the proper position in the beam of light by racking the microscope 

 draw-tube upward or downward in the usual manner by the coarse 

 and fine adjustments. The obser\ing objective is a special water 

 innnersion objecti\'e which makes contact witii the iii)per winilow 

 of the Biltz cell through the medium of a drop of distilled water. 



