42 The Microscope 



Negative Materials. The emulsion that is spread on a cellulose acetate 

 base to make film contains, besides silver bromide and gelatin, additions 

 designed to give each type of film its special characteristics. The char- 

 acteristics of interest to the photomicrographer are resolution, speed, 

 spectral sensitivity, and contrast. Many of these are cross-linked so that it 

 is, for example, impossible to obtain a film of both high speed and high 

 resolution. 



Resolution. The resolution of a film is just as important to a photomi- 

 crographer as the resolution of his lenses. It is obviously impossible to 

 produce, as an image composed of grains of silver, pictures of particles 

 of the same size as the grains. The grains of silver in the image can, in 

 fact, be regarded as the pigment in a paint. When this pigment consists 

 of extremely fine particles— as, for example, the carbon in the ink with 

 which this page was printed— extremely fine lines can be sharply repro- 

 duced. If, on the contrary, the ink had been made from coarse, sooty par- 

 ticles, the outlines of the letters would inevitably have been blurred. 



Speed. The speed of a film is the measure of its over-all sensitivity to 

 light. An image on a photographic film, just as much as an image on 

 the retina of the eye, may be brilliantly or dimly illuminated. In either 

 case there is a range of tones between the brightest and dimmest. A 

 photographic film must be exposed just so long as will permit the brightest 

 light in any particular image to penetrate the full depth of the emulsion 

 while the dimmest light is just affecting the surface. Very sensitive, or 

 fast, films require a short exposure while very insensitive, or slow, films 

 require a long exposure. The only reason for using fast films, which usu- 

 ally have relatively poor resolution, is to permit short exposures, and the 

 usual purpose of a short exposure is to "stop" moving objects. Slow films 

 of high resolution are therefore the only ones that need be considered for 

 photomicrography. 



Spectral Sensitivity. Different wavelengths of light have different ener- 

 gies. Short wavelength (blue) light beats like a staccato roll of drum- 

 sticks on the film. It exerts far more effect than the slow lapping of the 

 long waves of red light. It follows that an ordinary photographic emul- 

 sion is very fast to blue light and very slow to red light. Films of this 

 type are to all intents and purposes blind to red light and quite incapable 

 of distinguishing between blue and white. A microscope slide stained in 

 various shades of red and blue, photographed on such a film, would have 

 clear gelatin where there should be all the shades of red, and an almost 

 uniformly massive deposit of silver where there should be all the shades 

 of blue. A print from this negative would show the reds and pinks as 

 solid black and all the blues as just a pale gray shadow. These films are 

 obviously useless for photomicrography unless there is occasion to photo- 



