Photography to Micrometry. By Dr. Woodward. 147 
use* Nor was I siirprised to read, near the end of the second 
paper, that no one who has not tried it can form an idea of the 
difficulties attending the application of high powers to micro- 
photography, and a single tolerably good negative is often the 
only result of a hard day's labour." 
By my own easy method, on the other hand, an average 
morning's work produces from fifteen to eighteen successful 
negatives. The time of exposure with the objectives named 
above, arranged to magnify the blood - corpuscles about 1000 
diameters, is usually less than a second. A heliostat is therefore 
not necessary to obtain satisfactory results, as I have several 
times shown by taking such pictures quite as well without it as 
with it. The heliostat, however, is a great time-saver, since 
without it the light must be readjusted for every picture. I 
recommend anyone about to experiment in this direction to pro- 
cure one. An instrument which will answer every purpose can 
be extemporized for a few dollars out of a bit of looking-glass 
and the works of a Yankee clock ; but even the cost of the 
admirable heliostat of Silbermann, as made by Dubosq, is only 
five hundred francs, which will be more than repaid by the time 
saved during the first year, unless the time of the experimenter is 
not worth as much as that of an ordinary day-labourer. 
I also find it economical to employ a professional photographer 
in my dark-room, and I recommend others to do the same. By 
this plan the microscopist is left free to devote his sole attention to 
procuring the best possible optical images, while the photographer 
has nothing to consider but the best possible chemical work. A 
much greater quantity of good work can be produced in this way, 
in a given time, than is possible if the microscopist undertakes to 
do the photography himself. In this case he almost certainly 
sacrifices the optical part of the work to the photographic, or vice 
versa, and a reasonable degree of success is attained only by a 
wasteful expenditure of time. 
Satisfactory photographs having been obtained, the corpuscles 
are next to be measured on the glass negatives. Paper prints are 
never exactly the size of the negatives. They spread a little if rolled ; 
if not rolled, they may prove either a little larger or a httle smaller 
than the negatives. Moreover, this spreading or contraction does 
not take place equally in all directions, and is sometimes quite 
irregular. It is most accurate therefore to measure on the nega- 
tives. For this purpose I lay the negative, with its varnished film 
uppermost, on the ground glass of an ordinary photographic re- 
touching frame, illuminated from behind by a mirror, and measure 
* Since writing the above, I have seen a photograph of blood by Dr. Seller 
which is free from diffraction. It is of course taken with a much lower power 
than the others. Even this picture is not sharp, as it would have been had a 
proper method been used. 
