ADJUSTMENT TO LIGHT 73 



number of the exposure. The movement of the wheel is regu- 

 lated by a cUck. The metal case is made in two parts in such 

 way that the bottom may be readily removed and the strip 

 placed in position. The photographic paper used is the kind 

 called "solio." The photometer must be filled in the dark room, 

 or at night in weak light. A strip 6 mm. wide is cut lengthwise 

 from the 8x10 sheet, and J of an inch is cut from one end in 



Fig. 20. A new form of the simple photometer, showing the numbered 

 face and the slit for making exposures. 



order to secure the right length. A crease is carefully made \ of 

 an inch from each end to prevent the breaking of the paper when 

 the cork plug is put in place. The strip is then placed upon' the 

 wheel, great care being used not to touch the coated surface with 

 the fingers. It is fixed in position by using a piece of cork to 

 hold the creased ends in the slit of the wheel. The latter is then 

 placed in the ca.se, the zero turned until it is opposite the index 

 line, and the photometer is ready to l)e used. 



90. Making a standard. An exposure is made Ijy moving the 

 slide quickly, in such a way as to uncover the entire opening. 

 Care must be taken not to pull the slide completely out of the 

 groove, as it is impossible to replace it with sufficient quickness. 

 The length of exposure is determined by means of a watch. A 



