Ch. VII] PHOTOGRAPHIC ENLARGEMENTS 233 



§ 360. Size of condenser required. — The general law is that the 

 diameter of the condenser must be equal to or somewhat greater 

 than the diagonal of the negative or part of the negative to be enlarged. 

 For example to enlarge the whole of a lantern slide negative (85 x 

 100 mm.), the condenser should have a diameter of 14 cm. For a 

 negative 100 x 125 mm. the condenser should be 18 cm. in diameter; 

 for one 125 x 175 mm. the condenser should be 23 cm. in diameter, 

 and for a negative 200 X 250 mm. the condenser should be 35 cm. in 

 diameter. 



§ 361. Objectives to use for enlarging. — It is necessary to 

 use an objective which has been corrected for photography. The 

 ordinary projection objective gives a good visual image, but not a 

 good photographic image. The iris diaphragm must be wide open 

 (§ 289, 362). 



In preparing for printing, which of course is done in a dark room, 

 put some white paper in a printing frame with a clear glass in it. 

 Hold it in the path of the beam from the projection apparatus, and 

 either by moving a support near the apparatus, or by moving the 

 projection apparatus, get the desired size of picture. One can deter- 

 mine the exact magnification by putting a lantern slide of the metric- 

 scale (fig. 104) in place of the negative and projecting its image 

 upon the white paper in the printing frame. 



§ 362. Focusing and printing. — Focus the image of the negative 

 as sharply as possible. Then put over the end of the objective a 

 cover of some kind with ruby glass in it. This will allow the light to 

 pass in part, but it will not injure the photographic paper to be used. 



Place in the printing frame some developing paper like cyco or 

 velox. Place the printing frame in position. The image will show 

 clearly on the paper by the red light. When the frame is in the exact 

 position desired, remove the cap with ruby glass and make the ex- 

 posure. With an arc light the time will vary from about 2 to 10 

 seconds, depending on the density of the negative. Cover the objec- 

 tive, turn off the arc lamp,and develop the print as for contact printed 

 pictures. As shown in § 289, a mazda lamp may be used instead 

 of an arc light for enlarging. If the rather large source of light 

 in the no volt lamp is used, a diffuser of ground glass is needed 



