4 
Wenham, on Microscopic Photography^ 
the weather is warm develop with a half-grain solution of 
pyrogallic acid. 
For the lowest powers, it is oftentimes not necessary to use 
any arrangement for condensing the sunlight upon the object, 
the simple reflection from the plane mirror being sufficient ; 
but the half-inch object-glass, and upwards, requires a bull's- 
eye lens of about three inches in diameter. If the objects are 
small and delicate, such as the DiatomacecB, the achromatic 
condenser must be used in combination. I should mention 
that a piece of yellow glass must be let into the upper 
portion of the shutter, transmitting enough light to manipu- 
late by. 
Having briefly described the method of operating, I will 
state what I consider to be its peculiar advantages. The 
object is focussed with great facility and certainty. We are, 
in some cases, enabled to obtain an impression of an object 
mounted so as to be out of a flat plane ; in some instances, by 
the convenience afforded for inclining the sensitive plate at 
every angle relative to the optic axis, and in others by focus- 
sing two or more separate times. For example, suppose the 
object to be a Fly's foot, one of the pads of which lies in a 
lower plane, and is consequently out of focus ; while the im- 
pression of the first half is being taken, the next which is out 
of focus can be stopped off with a card ; the second focus is 
then taken, and the first impressed portion of the object ob- 
scured : thus by a diversified series of paper stops, we may 
approximately represent the entire form of an object, the 
whole of which cannot be seen, except by a series of three or 
four focal adjustments. 
A very great number of microscopic preparations are so 
organized as to be composed of parts which require different 
periods of exposure to produce a perfect impression ; some 
parts will be barely defined, while others are utterly solarised 
and lost from over exposure. The method that I have ex- 
plained affords especial facilities for stopping off the most 
tender portions. 
As it is requisite for the purpose of ensuring quick and 
accurate focussing, together with the most distinct impressions, 
that the actinic and visual foci of the objective should be coin- 
cident, I give the result of my investigations on this point. 
For the highest powers the difference is sometimes so small as 
to render the correction a matter of trifling importance ; but 
with the i inch, 1 inch and 1^ inch, the distance is very 
considerable ; in fact, the more perfect the object-glass for 
microscopic purposes, the less is it suited for a photographic 
lens. The object-glasses are invariably what is technically 
