120 THE MICROSCOPICAL NEWS. 
easy method of testing the time of exposure, four tests being ob- 
tained on one plate. Another method is to partially raise’ the 
shutter at intervals, noting the time for each exposure ; then to 
cut the plate down the middle with a diamond, and develope one 
half two or three minutes longer than the other. Six variations on 
the exposures can thus be seen, and the utmost novice will be able 
to discern “ which way the cat jumps.” 
Photographic Process.—lf sunlight were always at command I 
should prefer the wet collodion process, on account of the readi- 
ness and rapidity with which trial plates can be developed and 
examined. Every object differs so much in density or size that the 
time of exposure is ever varying, and carefully-repeated experiments 
are necessary. I have heard it stated that Dr. Maddox thought he 
did well to secure one good negative a day. ° 
Development.—For readiness of application at rare intervals of 
leisure I have preferred the ferrous oxalalate developer for dry 
plates, and all my gelatine negatives have been executed so far by 
this process ; but it is doubtful whether this method allows of so 
much latitude in nursing up the contrasts in a negative of a very 
transparent object, as is afforded by the pyro. developer. I have 
also intensified, where requisite, with the saturated solution of 
bichloride of mercury, and, after well washing, steeping in the solu- 
tion of ammonia. ‘The solution of mercury may be kept in stock, 
and used over and over again. 
Magnification.—The greater the disparity between the distance 
of the front lens from the object and the distance of the lens from 
the sensitive plate the less chance is there of securing penetration 
or deep focus. Therefore, if large prints are required, better re- 
sults will follow from employing a low power and taking a small 
picture, afterwards enlarging from the negative. Quarter-plates 
will suffice in most instances for this class of work, and those 
members who have seen Professor Piazzi Smyth’s negatives of the 
Pyramids, only one inch square, and enlarged three diameters by 
Mr. Pollitt, will be aware how well they bear further magnification 
on the lantern screen. 
The highest resolution I have heard of or seen by photomicro- , 
graphy is that by Dr. Woodward with Zeiss’s oil immersion lens 
one-twelfth of an inch, on the diatom Amphipleura pellucida, where 
the strie, which in nature count about 100,000 to the inch, are 
plainly delineated on the print ; and Mr. Crisp, the secretary of the 
R.MLS., tells me the lines have never been so clearly shown by 
ordinary vision. The nineteenth band of Nobert’s test-plate of 
finely-ruled lines on glass, containing about 100,000 to the inch, 
was also resolved by this unapproached operator with Tolles’ one- 
eighteenth of an inch immersion lens. 
From certain late researches of Professor Abbe the theory is 
