146 Miscellaneous Intelligence. 
as is used for the microscope, the chemical focus will be somewhat 
more distant from the object than the visual focus, and it therefore 
becomes necessary to make some allowance for this difference. 
This may be done in two ways, either by placing the sensitive plate 
somewhat farther off than the ground glass on which the image is re- 
ceived, or by altering the focus by the fine adjustment; the latter being 
the plan | prefer, as I find it much more accurate. ; 
The amount of difference between the foci probably varies in every 
ak y be asc 
A 4-10ths of an inch, about 2 divisions, or 1-1000th of an inch farther 
off. With the 1-4th, and higher powers, the difference between the 
abo 
discovery of Professor Stokes of the property possessed by certain 
transparent media of arresting the chemical rays. 
_ Any account of the preparation of the collodion, &c. &c. would be 
more fitted for a work on photography, and would render the present: 
paper much too ge: eover ther is abundance of informa- 
tion on photographic manipulatory details readily accessible in numerous 
publications, such r Hu al, Mr. B am’s, Mr. 
: . j &c. There are, however; 
one or two points which it is as well to allude to. If the film of a col- 
lodion picture be examined by the microscope, some specimens will 
present an appearance very much resembling condensed cellular tis- 
“sue, such as that seen in the cuticle of leaves, being apparently. 
up of flattened irregular hexagonal cells ; while others seem to consist 
of an entirely structureless amorphous mass; the latter sort of col- _ 
ion is most suitable for microscopic purposes, 7 
