512 EXPERIMENTAL PHARMACOLOGY 



inches wide by 18 inches long. They are made exactly like 

 the ordinary photographic printing frames and a glass plate 

 is placed in the front of the frame. Against this plate the 

 varnished black tracing (which should have been smoked 

 thoroughly black) is placed (facing outward). Over the 

 tracing a suitable sized piece of blue print paper is placed 

 (this is done in the dark room) facing outward, i. e., the 

 blue side of the paper rests against the back of the tracing. 

 The back is clamped into the frame and the bright sunlight 

 is allowed to shine through the glass for a period varying 

 from five to fifteen minutes depending on the quality of 

 the blue-print paper, the character of the black tracing and 

 the brightness of the sun. With a very bright sunlight 

 and first class printing paper the exposure should not ex- 

 ceed ten minutes on the average (and may often not require 

 over five minutes). But much blue-print paper on the 

 market is not this sensitive. Poor paper should be scru- 

 pulously avoided. Only experience can serve as a very re- 

 liable guide to one in determining when an exposure has 

 been carried far enough. The statement is made that, for 

 ordinary photographic negatives, the printing should be 

 carried on until the shadows, that is, the darkest parts of the 

 picture, assume a bronzed appearance. In printing kymo- 

 graphic records the printing should be carried on until the 

 print looks considerably darker than one would at first sus- 

 pect to be necessary, for the washing removes some of the 

 coloring material. 



As soon as the printing has gone far enough the pape~ 

 is taken out and washed thoroughly in clear water. The 

 print is then hung up to dry after which it can be properly 

 trimmed and pasted in the note book. 



It is strongly advisable for some one who is employed by 

 the laboratory (technician, etc.) to make up all blue-prints 

 required by the students. These prints can be sold to the 

 students for about one-half or one cent apiece (which prac- 

 tically covers the cost of the paper), and the proceeds of the 

 sale should go to the person who makes the prints. In most 



