814 
PROCEEDINGS OF THE SOCIETY. 
Mr. C. Beck, calling attention to a recent discussion as to the method 
of obtaining a standard tube-length for the Microscope, pointed out that 
the difficulty experienced by some in fully understanding the matter 
arose from the way in which English optics had been taught — namely, 
that the distance of the focus should be measured from the centre of a 
lens. This was laid down by all English text-books, and it was upon 
this point that the whole thing turned, for although in the case of a 
single very thin lens the difference would not be much, if they took their 
measurement from the centre of a combination they would get a result 
materially in error. Having drawn explanatory diagrams upon the 
board he showed that the plane varied according to the kind of combina- 
tion used, and that whereas in an ordinary English 1/4-in. objective the 
principal plane was somewhere about the back lens, in a 4-in. objective 
it would be about 2 in. up the tube, so that the tube-length was in this 
way practically decreased, and in order to give the tube a more uniform 
length, a 4-in. objective would have to be made with a very long 
mounting, and high powers in similarly short ones. It would hardly 
meet the case, however, to mount high-power objectives in very short 
tubes and low powers in long tubes, especially as in using low powers 
with the binocular it was necessary to have the back lens as near as 
possible to the binocular prism in order to get a good light in both tubes. 
Dr. W. H. Dallinger said that Mr. Beck had called attention to a 
subject which certainly deserved great attention, and there were points 
about it which called for the most careful consideration. He had 
himself quite come to the conclusion that it would be a wise thing for 
the Society to go into the whole question, for there was very much more 
in it than could be done upon the board, though he could but con- 
gratulate Mr. Beck upon the able way in which he had brought it before 
them. They would find that the subject was one which could not be 
measured and weighed by persons who had not given themselves to the 
study of the questions involved, neither did he think that opticians 
themselves could compass all that it comprised. In short, he thought 
they wanted a committee appointed to follow out the whole question. 
The President expressed the thanks of the Society to Mr. Beck for 
raising this very important and interesting question. The subject was 
one well worthy of the attention and consideration of the Fellows of 
the Society. Any suggestions which they might have to make on the 
subject would be gladly received by the Council. 
The following Instruments, Objects, &c., were exhibited 
Mr. C. Lees Curties : — New model of a Microscope by Leitz of 
Wetzlar. 
Mr. C. Rousselet: Notops brachionus and Stephanoceros EicJihornii , 
mounted. 
New Fellows : — The following were elected Ordinary Fellows : — 
Messrs. H. Neville Beeman, Arthur Blake, Edward Russell Budden, 
Dr. George Alexander Cohen, Messrs. Oliver Collett, Charles Lees 
Curties, Robert W. Dunham, Dr. Elmer Ellsworth Hagler, Messrs. 
Edwin Ernest Hill, Samuel Lowe, and Dr. Ernest Wende. 
