MEMORANDA. 75 
with this cement, it is placed upon the glass slip, the cover 
put over it, and gently pressed down, to insure close contact 
without squeezing the cement out of the leather. It may 
then be put away. The cement soon dries, and at any sub- 
sequent time the superfluous leather may be cut away close 
round the edges of the thin glass cover. 
_ Care should be taken that the leather or paper is soft, and 
free from elasticity, so that it may lie flat upon the glass; 
also that it has enough cement to insure adhesion, and not 
so much as to spread over the object when the cover is placed 
upon it. The consistency of the cement also requires atten- 
tion ; it should be just so fluid that it is readily absorbed by 
blotting-paper. 
So far, I have found it combine the advantages of speed, 
ease, certainty, and cheapness, requiring no special appliances 
in its performance,—the liquid glue, bottle and brush being in 
constant use for other purposes, where cement or a yellow 
varnish is required, and the punch being the same that is 
used for cutting out labels, &c. The little squares of leather 
for thick objects, and of paper for thin objects, are all that 
it is necessary to keep specially for this purpose.—B. S. 
Proctor, 11, Grey Street, Newcastle-on-Lyne. 
Plan for finding the Focal Length of Objectives.—If you 
think the following communication worth insertion, per- 
haps you will find it a place in the ‘Journal’ It is a 
modification of Professor Thury’s plan for finding the 
focal length of microscopic objectives. It was suggested 
by reading Captain Mitchell’s paper im the October 
number of the ‘ Journal,’ and may be of use to those who, 
like myself, wish to know the true focal length of their objec- 
tives, but have not a positive eyepiece micrometer. The differ- 
ence from the professor’s plan consists in using the eye-lens of 
the negative eyepiece only, the micrometer being placed, as 
usual, exactly in its focus. I send you a table of the results on 
my own objectives, also the magnifying power as given by the 
camera and my longest eyepiece, to prove the correctness of the 
method by comparison with the magnifying power obtained by 
multiplying the power of the objective by that of the eyepiece, 
the results being as coincident as unavoidable errors will allow. 
To arrive at this, I have, however, been obliged to differ from 
Captain Mitchell in considering the power of the objective to 
be the exact number that one of the stage covers of the eye- 
piece micrometer, and not with the addition of one, as in his 
method. Any one following out this plan will find that, as 
the power of the eyepiece must be the same with every ob- 
