Photomicrography 57 



photomicrography can do no more than record the total light emerging 

 through the ocular. The proportion of this that is transmitted through the 

 specimen depends on the thickness and method of staining of the latter. 



Using a film-camera instrument, it is best to take a few frames at vari- 

 ous exposures and to select the best. Figure 53, for example, shows the 

 strip of film used in preparing Figs. 135 and 136. This is both inconvenient 

 and expensive on larger films held in slide holders. In this case the dark 

 slide should be manipulated so as to expose successively the whole, 

 three-quarters, one-half, and one-quarter of the plate. Most fine-grain 

 panchromatic films have a great deal of latitude— that is, will produce a 

 printable negative with a wide range of exposures— and most people keep 

 exposures on test strips far too close together. An exponential series of 

 three is perfectly satisfactory. If, therefore, experience suggests that one 

 second is the correct exposure, test pictures should be made at one-third, 

 one, three, and nine seconds. It is then easy by visual extrapolation to 

 arrive at the right figure. A test strip at one, two, three, and four seconds 

 would be relatively valueless. 



There is no substitute for experience in judging which of the developed 

 tests is correct. It is necessary, however, to remember that black and 

 white are never present. The lightest area in the ngative should therefore 

 have some faint trace of gray. The darkest area should transmit some light. 



Steps in Taking a Photomicrograph. If what has gone before has been 

 understood, what follows is easy. 



1. Set up microscope (Chapter 2) to give the best possible visual 

 image. 



2. If necessary insert into the system a color filter that will produce 

 the required emphasis. 



3. Transfer the image, by direct projection, reflux, or split beam onto a 

 photographic film. 



4. Prepare a test strip and develop for the time, at the temperature, 

 and in the solution recommended by the manufacturer. 



5. On the basis of step 4, make the final picture. 



All this is just as easy as it sounds. Photomicrography is only the 

 mutual application of a sound knowledge of photography to a sound 

 knowledge of microscopy. With these prerequisites, it is easy; without 

 them it is impossible. 



