Photomicrography 213 



ing the subject, but uot for critical focvisiug. For maximuui sharpness, 

 use the clear window method. Make a fine X mark on the ground 

 glass with India ink, on the diagonals ot the glass. Allow the ink to 

 dry, place a drop of balsam or cover glass resin on the mark and 

 lower a cover glass on the resin. This will make a clear window in 

 the ground surface. A focusing glass may be purchased, but an in- 

 expensive one can be made by fitting a 3 to 5 X magnifier into a 

 metal tube of such length that when the tube rests on the clear 

 area of the ground glass, the X mark is in focus. Bring the image 

 into approximate focus on the ground surface, view the image 

 through the magnifier and bring into sharp focus. 



Exposure Meters 



An extensive literature has accumulated on the subject of ex- 

 posure control in photomicrography. The most accessible sources are 

 the indexes of Stain Technology and the Journal of the Biological 

 Photographic Association. Only a brief survey of the principal me- 

 thods can be given here. 



Extinction meters have been used by skilled photomicrographers, 

 who could probably standardize an apparatus and make good nega- 

 tives without a meter. The photoelectric meters used for general 

 photography will register a significant reading with some photo- 

 micrographic apparatus. This makes possible the calibration of the 

 apparatus and fairly satisfactory exposure control. 



Several highly sensitive, but expensive, electronic meters are 

 available. These meters give good readings in the plane of the emul- 

 sion, in fact permit probing of small areas of the image. Consult the 

 advertisements of scientific journals for the currently available meters. 



If an exposure meter is not available, an experienced photographer 

 who can judge negative densities can obtain good negatives with a 

 little expenditure of film and time. Assume that previous experience 

 with a certain magnification suggests an exposure of 15 seconds. Draw 

 the dark slide halfway out of the film holder and make a 10-second 

 exposure. Remove the dark slide and expose for another 10 seconds. 

 The two halves of the film have had 10 and 20 seconds respectively. 

 Develop the negative and decide whether the next exposure must be 

 less than 10 seconds, more than 20 seconds, or an intermediate interval. 



Negative Materials 



Orthochromatic emulsions can be used for photomicrography. 

 These emulsions are sensitive to green, blue, and ultraviolet. A black 

 object or one that is rich in green or blue may be rendered accurately 



