654 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
opposite qualities were used, uo amount of careful exposure or develop- 
ment would give a negative having sufficient contrast to print properly. 
Similarly, with an object having great contrasts, a plate giving little 
contrast and long range of tones, will give a negative in which the con- 
trasts of the object are so lessened that printable details are given in the 
densest parts, while were a plate having opposite qualities used, the 
strong contrasts of the object would be so reproduced or exaggerated that 
a print devoid of all detail could be obtained only. As in photomicro- 
graphy, owing to the peculiar nature of the objects to be photographed, 
great difficulties are often encountered, the ingenuity of the operator 
often being taxed to the utmost, it follows that a proper selection of the 
plate to be used will add greatly to his resources, and will enable him to 
obtain results which could not be obtained were only one make of plates 
used, whatever legerdemain of exposure or development he might 
practise. 
But, in order to take advantage of the different properties of different 
plates, it is necessary to know exactly how they differ ; and this must be 
determined not by exposing the plates to be compared in a camera where 
the light may be constantly varying, and where the personal equation of 
the operator may enter as a disturbing factor, but in a manner by which 
each shall receive equal treatment. For purposes of comparison I have 
used a pad of tbin white tissue paper (onion skin), 4 in. by 4J in. in 
size, made of superimposed pieces of paper, each sheet being 4 in. long, 
and 1/2 in. narrower than the next sheet underneath. This pad, when 
placed on a piece of clear glass in a 4 in. by 5 in. printing frame, and 
viewed by transmitted light, gives nine gradations of density, from clear 
glass up. Such a pad answers for all practical purposes, though one 
7J in. long, placed in a 5 in. by 8 in. printing frame, and used with 
strips cut lengthways from 5 in. by 8 in. plates, will give a longer range 
of gradations. To test two or more plates, a strip about 1 in. wide and 
5 in. long is cut from each, and placed side by side, film side down, on 
the pad in a 4 in. by 5 in. printing frame. They are then clamped in 
the fiame, exposed for one instant to diffuse daylight, or for a few 
seconds to lamplight ; and are then all developed together in the same 
developer. 
It is best to develope for fully twenty minutes in a covered tray, 
with a developer containing a rather large quantity of sodium sulphite. If 
about thirty grains of the granular sulphite is used to each ounce of the 
developer, yellowing of the films, which might be produced by the pro- 
longed development, will be prevented ; and this without any ill effect 
on the resulting negatives. Development for fully twenty minutes is 
recommended in order that development be fully completed, i. e. that 
all the molecules of silver acted upon by light be reduced, for in this way 
only can the exact properties of all the strips be brought out, inasmuch 
as some plates develope more rapidly than others, and a stoppage of 
development before completion will produce erroneous results. The 
illustration is a reproduction of the result arrived at by comparing a 
“Harvard” plate, sensitometer 40, with a “Seed” plate, sensitometer 
25, in the manner above described (fig. 73). It is a reproduction of 
the negatives themselves (not a print from them), so the lighter bands 
represent the thinner bands of the original negatives. 
