CH. X] PHOTOGRAPHY AND PROJECTION 381 



For the occasional use of a laboratory the stray light can be ex- 

 cluded by means of asbestos paper. Sometimes the arc lamp is 

 put on the outside of a partition, but that necessitates going out of 

 the printing room to adjust the lamp. If direct current is available 

 an automatic lamp is a great convenience. 



PHOTOGRAPHING WITH PROJECTION APPARATUS 



541. Apparatus which will give good projection of micro- 

 scopic specimens can, with slight modifications be used for photo- 

 micrography. 



There are three possibilities: 



(1) Printing the image directly on a developing paper. 



(2) Exposing a dry plate directly to the image as for the paper. 



(3) Using a camera and plate holder. 



542. Printing the projected image directly on a developing 

 paper. With the apparatus set up exactly as for drawing one can 

 expose a sheet of developing paper to the sharply focused image of 

 the specimen as described for the enlargement of negatives ( 532). 

 The lights and shades will be reversed, but all the outlines and 

 details will be present. This is a convenient method of getting an 

 enlarged record of the specimen. 



It is also a good method for making pictures for models ( 511) 

 especially when there are many details. With the cheap develop- 

 ing papers in rolls now obtainable the expense is not greater than 

 for making drawings, and there is liable to be a gain in accuracy. 

 The main draw-back is that but a single picture is made of each 

 specimen for a single exposure, while in drawing it is as easy to 

 make two or three as one, by using carbon paper ( 511). 



543. Exposing a dry plate directly to the image. A dry plate 

 may be exposed as just described for the developing paper. The 

 object must be so placed on the stage of the microscope that the 

 image on the screen will be a mirror image of the specimen, that is, 

 the rights and lefts will be reversed as they should be in a negative. 

 The image is sharply focused, and the light cut off with a deep red 

 glass so that the plate will not be affected. 



