1890.] MICKOSCOPIOAL JOUENAL. 235 



" back," or, in other words, with the ground surface of the ordinary 

 screen. With such an arrangement, or a modification of it, it is easy 

 to obtain sharp definition with so high a power^that the image would 

 be absolutely invisible on the ordinary screen, owing to the loss of light 

 and amplification. 



The time of exposure will vary with the intensity of the source of 

 light, color, and density of the object, sensitiveness of the plate, and 

 the magnification of the object. By using plates of one degree of sen- 

 sitiveness and always using the light as nearly as possible under the 

 same conditions, two of these variables can be eliminated, and by re- 

 membering that roughly the time varies as the square of the linear am- 

 plification, an idea can be formed of the proper exposure. 



It would be highly desirable that some standard should be arrived 

 at, but the difficulty of determining a standard of light has not yet been 

 fully overcome. 



The nearest approach to any refinement of the kind is that described 

 in a "' Manual of Photo-micrography," by Dr. Bousfield, and involves 

 the use of Warnerke's sensitometer screen. 



This screen, which consists of a series of spaces numbered from one 

 to twenty-four, containing smoke-colored tissues of regularly increasing 

 density, by application to the focussing screen, affords a guide to the 

 intensity of the light falling on it ; and the method of use is to read the 

 last number visible, and, knowing the rapidity of the plates in use, refer 

 to a table which has been deduced from a number of experiments. 



In color experience alone must guide ; reds and yellows, as a rule, 

 are favorable to photography, while blue and violet, which are trans- 

 parent to the actinic rays, are difficult to obtain satisfactory negatives 

 from. 



Any of the well-known brands of plate may be used, either ordinary 

 speed or rapid, but there is no advantage in the use of rapid plates with 

 low powers, say, up to half an inch ; after that, exposures begin to be- 

 come tedious, even with rapid plates, so much so that any one doing 

 much work requiring high powers would have to obtain an illuminating 

 source of higher actinic power, such as the electric arc or oxyhydrogen 

 lime-light. 



Magnesium can also be used, but is rather costly, and there is much 

 difficulty in dealing with the smoke from it and in keeping the glowing 

 point accurately in the focus of the condenser. 



Sunlight has even been used with success by Dr. Woodward and 

 with the heliostat to obtain a constant beam, good definition has been 

 obtained up to 5,000 diameters. 



The exposure made, the plate is taken into the dark room and there 

 developed. 



The plate, which is a square of glass coated with gelatin, holding in 

 suspension various haloid salts of silver, is, after exposure, to all ap- 

 pearance the same as it was before, but a subtle change has taken 

 place, for all those parts of the plate upon which light has fallen during 

 exposure are now highly sensitive to the action of certain reducing 

 agents, many of which might be employed, but practically we have to 

 choose between three processes, viz., ferrous oxalate, pyrogallol, and 

 quinol. 



The developer is poured on to the plate, and the latter is watched in 



