ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
389 
then finely on to the ground glass of the camera by means of the 
fine-adjustment screw b. Use a large diaphragm on the Microscope 
base. Expose in the ordinary way. A little practice will soon show 
the right exposure to be given, always using the same lamp. A small 
beading round the top of the table holds the camera firmly. 
(5) Microscopical Optics and Manipulation. 
Determination of “ Optical Tube-length.” * — The following paper 
on this subject was read by Mr. A. Ashe before a recent meeting of the 
Quekett Microscopical Club : — 
“ This is one of those practical matters the investigation of which 
many microscopists postpone indefinitely, and generally end by neglect- 
ing entirely, under the mistaken impression that its solution is involved 
in much difficulty, requiring an advanced knowledge of the laws of optics 
and a large amount of manipulative dexterity in order to arrive at a 
satisfactory result, and that even if a correct measurement can be made, 
the information so obtained is of no real value to the worker. The 
fallacy, however, of this latter view is so obvious, that it needs no 
refutation to any one who has taken the trouble to estimate the magni- 
fying power of his own instrument. 
To those who are content to accept the figures given in an optician’s 
list as to the amplification of their various lenses, the following quotation 
from Mr. Crisp’s well-known article f may carry some weight : — 
‘ Microscopists have always recognized that the length of the tube of 
a Microscope is a factor in determining the amplification of the image, 
that the amplification is generally greater with a 10-in. tube than with 
one of 6 in., and that we obtain an increase of power by pulling out the 
draw-tube. Here, however, all exact notions as to the functions of the 
tube-length have practically stopped, so much so that there has not been 
any agreement even as to how the length of the tube is to be measured, 
whether from the front or back lens of the objective to the field-lens, 
the diaphragm, or the eye-lens of the eye-piece.’ 
Since these lines were written, now some eight years ago, it has 
come to be very generally admitted that the optical tube-length must be 
measured from the posterior principal focal plane of the objective to the 
anterior principal focal plane of the ocular. 
But the question obviously arises, where are these focal planes 
situated, how are their positions to be located, and the distance between 
them estimated? 
The desire for information on these points will certainly not be 
rewarded by any light the average microscopical textbook may throw on 
the subject, for, whilst laying stress upon the relationship existing 
between tube-length and amplification, they generally leave the reader 
very much to his own resources as to the methods employed in solving 
the former part of the problem. 
A recent article in the Journal of the Royal Microscopical Society 
(1892, pp. 545, 546) on this subject is very interesting, but un- 
fortunately the method suggested, whilst perfectly accurate and 
* Journ. Quekett Micr. Club, v. (1893) pp. 152-4. 
t This Journal. 1883, pp. 816-20. 
