14 THE MICROSCOPICAL NEWS. 
ON “OPTICAL TUBE-LENGTEH: 
By Frank Crisp, V.P.L.S., Sec. R.M.S. 
(Read 14th November, 1883.) 
ie is not a little strange that at this late period in the develop- 
ment of the Microscope, an element of capital importance both 
from a theoretical and a practical point of view should have been 
left entirely unconsidered, and indeed unknown ; and the fact that 
it is so, illustrates the disadvantage which English-speaking micro- 
scopists have always been under in having no text-book dealing 
with the theory of the Microscope. 
In a letter written more than a year ago in reference to the: 
Table of Magnifying Powers published in the Journal, Professor 
Abbe called my attention to the erroneous notions which prevailed 
on the subject of the magnifying power of the Microscope, and 
which he had been the first to clear up,* and I ought then to have 
published the explanation now given here, but the pressure of 
other engagements diverted my attention, and I confined myself to 
explaining the matter verbally to those who attended the meet- 
ings. Finding, however, that the Committee on Eye-pieces of the 
American Society of Microscopists have been misled by the Table 
in question, it is obviously desirable not to delay the explanation 
any longer. 
Microscopists have always recognized that the length of the 
tube of the 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 function 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. 
In particular, no view of tube-length has been held which would 
explain the following apparently paradoxical statements :— 
That two objectives of precisely the same focal length used with 
the same tube and the same eye-piece may nevertheless give diffe- 
rent magnifying powers. 
That two objectives of different focal lengths used with the same 
tube and eye-piece will not give mangnifying powers in proportion 
* Professor Abbe also communicated it to Dr. Dippel, by whom it was em- 
bodied in the last edition of ‘ Das Mikroskop,’ 1882. 
