Photomicrography 43 



graph a faint pink smear, the gradations of which are almost impercepti- 

 ble to the eye. 



Various additives to silver bromide emulsions cause them to become 

 sensitized to red. These films are called "panchromatic," often shortened 

 to "pan," since they can accurately reproduce gradations of all colors in 

 gradations of gray. Bright red and bright blue will both be pale gray; 

 dark red and dark blue will both be dark gray. It is easier to balance 

 the films at the ends of the spectrum than in the middle, so that even 

 the best of them see bright green as a darker shade of gray than bright 

 blue or bright red. This can be compensated, if the exact rendering of 

 shades of color as shades of gray is vital, by inserting a light-green filter 

 somewhere in the optical system. This filter appears green because it 

 absorbs part of the red and blue components of the light but transmits 

 all the green. This intensifies the green components of the image, which 

 thus are reproduced by the film in tones of gray equivalent to the blue 

 and red components. Colored filters are also widely used in photomi- 

 crography deliberately to intensify one color at the expense of another. 

 This will be discussed later. 



Contrast. There is no pure black or pure white in the world. Both words 

 are relative terms used to describe the opposite ends of a long scale of 

 grays, each of which blends imperceptibly into the one next to it. Con- 

 trast is a measure of the length of this scale or of the number of steps 

 of which it is composed. Imagine the scale of grays in a print to be a 

 slope extending down from very bright to very dark. If this slope is very 

 steep, with few steps, it can be descended in a series of harsh and memo- 

 rable leaps. If the slope is gentle, with many steps, the descent is smoother 

 and less dramatic. Exactly the same effect is produced by high- and low- 

 contrast photographs. The high-contrast picture (Figs. 36 and 37) is 

 dramatic and eye-catching— qualities not to be despised in illustrations 

 of books and scientific articles. The low-contrast picture (Fig. 69) is a 

 more truthful representation of the original. The photographer must 

 decide for himself whether his picture should record an observation or 

 tell a story. 



Contrast may be controlled by the selection of negative material, in 

 processing the negative material, and by the selection of the positive 

 material on which the print is made. The last is the best. Extremely con- 

 trasty negative material should only be used to take pictures of objects, 

 such as unstained cells, that lack contrast in themselves. 



Selection of Negative Materials. It will by now have become apparent 

 to the reader that the ideal film for general photomicrography is a high- 

 resolution, slow-speed, panchromatic film of medium contrast. The three 

 best known of the films are Kodak Plus X, Ansco Supreme, and Adox KB 



