ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
169 
minutes will be required if D be substituted for it, which magnifies not 
quite twice as much. B, on the other hand, will require but 15 seconds, 
and A only 4 or 5. 
The length of exposure will, of course, vary greatly with the quality 
of the light obtainable, but, as long as one has sufficient to see the object 
for ordinary purposes of microscopic observation, it is merely a question 
of so many minutes or seconds exposure more or less, and it is as easy 
to get a good photograph in an English November as in the brightest 
day of summer. 
The correction necessary for each combination requires, of course, a 
few careful experiments ; but this effected, is done once for always ; 
and, as each experiment helps greatly towards the next combination, 
one soon obtains a table of corrections for all the combinations one is 
likely to require. 
The photomicrographic expert will doubtless object that it is better 
to dispense with the eye-piece,* and employ the image given by the 
objective directly ; but though this may be so in the case of difficult 
objects, such as diatoms under high amplification, it necessitates the use 
of a camera of unwieldy length, and is, moreover, an almost hopeless 
task, unless the tube of the Microscope be done away with altogether, 
and the objective be made to screw directly on to the camera-front ; 
for, however carefully the tube may be blacked, the reflection from its 
sides produces an unbearable glare in the centre of the plate, and, in 
the case of a Microscope in regular use, the portion in contact with the 
eye-piece becomes so polished, that little else but central glare is visible 
on a negative given by the objective alone. For the purposes, there- 
fore, of the working biologist, it is far better to use the eye-piece, in 
spite of the small theoretical disadvantages of the plan. 
If the camera-length beyond the eye-piece be about 8 in., the scale 
of the resulting negative will be somewhere about the nominal mag- 
nifying-power of the combination in the case of Continental outfits. 
English opticians, however, calculate their amplifications at 10 in. 
from the eye-piece ; so that, in the case of English Microscopes, the 
amplification will be a good deal less than the nominal power of the 
combination. 
I cannot, however, recommend working with any greater extension, 
as very few lenses will bear the test. 
The plates used should be the slowest obtainable. The more rapid 
the plate, the coarser is the grain of the film, and therefore the less 
suited it is for our purpose. In this respect, gelatin plates compare 
ill with the old wet collodion, the grain of which was so fine that a 
sheet of the c Times ’ reduced to a square about half an inch square 
could still be easily read under a sufficiently powerful lens. 
Collodion emulsions are, however, coming a great deal into use 
once more for a variety of purposes ; and, though slow, would be 
doubtless to be preferred for photomicrographical w r ork. 
A waiter in the current c Photographic Almanac ’ praises highly the 
Hill-Norris collodion-plate (medium speed) for photomicrography ; but 
* [The general practice of the best photomicrographers in this country is to use 
an eye-piece.] 
