EXPOSURE-METERS. 143 



The correct time of exposure for photomicrographs varies so greatly with the size 

 of stop, length of bellows, kind of slide, number of objective, quality of light, rapidity 

 of plate, etc., that no very definite rules can be laid down, the right time in special 

 cases in Washington varying all the way from several minutes to ^-L of a second. 

 If the bellows-length is doubled, of course the time of exposure nmst be quadrupled. 

 IvOw powers, and especially Planars, let through a great flood of light and require 

 correspondingly short exposures. With low powers and sunlight the student might 

 begin on ^'5 second. With an oil-immersion lens and bright light he might tr}- 

 Y second or -} second. If the section is densely stained, much allowance must 

 be made for that. It is well, at least for a time, to keep a record book of subjects 

 and exposures to refresh one's memory. It saves the spoiling of many plates. Such 

 a record should include subject, length of exposure, stop used, objective and eye- 

 piece used, length of bellows, distance of the condensing lens from the Abbe con- 

 denser, time of day, time of year, quality of light, kind of screen, kind of stain and 

 density of section, kind of plate, developer used, time required for development, and 

 quality of negative, viz, overexposed, underexposed, or correctly timed. 



For outdoor work, and also for natural-size or .slightly magnified indoor work, 



a good exposure scale is sometimes usefial. The best ones known to the writer are 



the Wynne and the Wager. Success with the Wynne depends on one's judgment as 



to the proper changes in a good sensitive paper ; with the Wager it depends on one's 



judgment as to the quality of the light in the sky. After a little experience very 



uniform and excellent results may be obtained with either. Personally, the writer 



prefers to use the Wager (fig. 128), because it is simpler and takes less time. No 



scale is always to be depended on, there are so many variations in light and so many 



unprovided-for contingencies. Experience is after all the best guide, but until one 



has obtained it, genuine aids are not to be neglected. The beginner should first 



become familiar with the right exposure for one stop and one kind of plate, e. g., 



stop f 16 and Seed's 27, with a given bellows length. Having learned correct 



exposures under these constant conditions, it will be comparativel}- easy to change to 



other makes of plates and to other f stops. Slow isochromatic plates require 10 to 12 



times as long exposure as fast plates. In the matter ot stops the length of exposure 



is, of course, quadrupled every time the f stop number is doubled, and quartered 



every time it is halved, e. g., if stop 16 will give a perfect negative with one second 



exposure, stop 8 will require one-fourth second and stop 32, four seconds. Under 



the same conditions, stop 4 will require one-sixteenth second, and stop 64 sixteen 



seconds, and so on. With the Universal stops (those commonly used on the .shutters 



made in this country' and England) the exposure is doubled for the next higher stop 



and halved for the next lower one, instead of quadrupled or quartered, as in the case 



of the f. stops. 



For lantern slides the writer converts a small room into a camera box (plate 

 18). This room has a floor space about 6 by 5 feet. It has a north window 

 and a west window. Each window is provided with a double set of roller curtains, 

 the outer made of yellow cloth, the inner of a ver}' dense black cloth known in the 

 trade as double-faced, opaque, black shade-cloth, which lets scarcely any light through. 



