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 ^^ of a second. 

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

 Low 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 ^ second. With an oil-immersion lens and bright light he might try 

 j 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 useful. 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. 1 6 and Seed's 27, with a given bellows length. Having learned correct 

 exposures under these constant conditions, it will be comparatively 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 

 1 8). 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 very dense black cloth known in the 

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



