62 
POPULAR SCIENCE REVIEW. 
dial, or of a revolution ; a ± Smith & Beck required -fa of 
a revolution ; a Ross’s A~inch req nired of a revolution; and in 
a Powell & Leland’s -J^-inch the visual and actinic foci were 
found practically to coincide. These are but rough guides. The 
same powers by the same makers differ widely in this respect, 
and nothing but direct experiment in each case can decide the 
amount of correction required. As if these difficulties were 
not sufficient, it has been asserted that each separate light re- 
quires a different degree of correction; but in our experience 
the difference, if any, for oil, magnesium, and oxy calcium 
lights is so small that it may safely he disregarded. 
Screw Collar . — One small, but not unimportant, matter still 
requires to be noticed before proceeding to take the picture. 
In using the higher powers, the screw collar will be found a 
constant source of perplexity, and it is not easy to give any 
clear rules for its adjustment. Perhaps the best plan is to 
ascertain the most advantageous position with a low eye-piece 
before arranging the camera, and then to take the picture at 
that; of course, allowing as before for the amount of correction 
which the lens requires. If the result is not quite satisfactory, 
some attempt may be made with the screw collar to improve 
the focus on the screen. 
Of the photography I shall say little or nothing, as it does 
not materially differ from the processes in ordinary use; but I 
may observe that, if care, and cleanliness, and delicacy are 
requisite for ordinary work, they are infinitely more necessary 
where lines numbering fifty or sixty in the j-oVu P ar ^ °f an 
have to be represented clear and distinct on the film and trans- 
ferred to paper. 
The operator must tread lightly, or the focus will be dis- 
turbed ; he must be still, or dust will rise ; the smallest things 
must constantly be attended to, or the results will not equal his 
just expectations. 
I shall conclude this paper by shortly going through the 
process of taking a photograph of any object, say one of the 
finer Diatomacese. 
The camera is arranged, I will suppose, as represented in the 
plate — of such length as will give an amplification of 400 
diameters with the objective. 
First, centre the oil-lamp. Remove carefully the apex of 
the triangle (F) from the microscope, and let it rest on the base- 
board at the side ; take out the velvet from the tube of the in- 
strument, and insert the eye-piece. It will now be easy to fix 
on the object; bring it to the centre of the field, and obtain the 
best possible definition by adjustment of focus, screw-collar, 
and condensers. When this has been done, the eye-piece is 
again removed, the velvet tube replaced, and the whole made 
