3 So PHOTOGRAPHIC ENLARGEMENTS [Cn. X 



then the image is carefully focused by putting some white paper 

 on the screen or by having a ground-glass in the middle of the 

 screen. Then the objective is covered with a dark cap or with a 

 cap containing ruby glass, and the photographic paper is fastened 

 in place by thumb tacks or in some other way, care being taken to 

 stretch it smooth. 



538. Exposure. When the paper is in place the cap is 

 removed from the objective and the projected image will print on 

 the paper. The time necessary depends upon the magnification, 

 the density of the negative, the intensity of the light and the sensi- 

 tiveness of the paper used . It usually takes about one-fourth the time 

 one would print by contact using a 1 6 candle-power frosted incan- 

 descent lamp. A good plan is to try a small piece of the paper and 

 determine the correct exposure before printing on the large sheet. 

 After the exposure the objective is covered with the cap and the 

 paper is developed exactly as for contact printing. 



539. Diaphragm of the objective. In printing, the diaphragm 

 of the objective is wide open if the unmodified cone of light is used 

 for illumination. This has one defect with the arc lamp. If there 

 are any irregularities in the negative, such as minute scratches, etc., 

 they would show in the print, whereas if the illumination were from 

 an extended instead of a very small source like the crater of the arc 

 lamp, the slight defects would show very much less. 



To obviate this defect with the arc lamp one or more plates of 

 ground-glass or of milk white glass are placed in the path of the 

 beam before the negative. It must be put far enough from the 

 negative so that the grain of the ground-glass will not show. 



With the ground-glass or the milky glass in the beam the dia- 

 phragm of the objective can be closed as much as desired. The 

 use of the ground-glass and the closure of the diaphragm will, of 

 course, necessitate a longer exposure. 



540. Avoidance of stray light. If one is to do considerable 

 printing with the projection apparatus a light-tight lamp-house 

 must be used and light-tight bellows between the condenser and 

 the negative and objective. A special camera is most satisfactory. 



