

NOTICES OF MEETINGS, 291 

gentleman has given the subject some consideration ; and he is certainly better 
qualified than I am to give Mr. Napper the information asked for. 
Dr. WEBBER: You pay mea very great compliment which I don’t deserve, 
for although I fancy I have aclear idea of what is intended by numerical aperture, 
I don’t know that I have the ability to make it clear to others. It is one 
thing to understand a matter, but another to make it clear to others. As faras I 
have been able to catch it, it means that a quantity of light passing through a 
certain medium may be greater or less. That, so far as I can understand, is 
that a volume of light passing through air is scattered the same way as a very 
fine gas might be scattered, while if passed through oil or water in the same 
space a greater quantity of rays of light is passed. It is perfectly impossible to 
get 180 degrees of angular aperture, because you have no focal point. Light pass- 
ing through air in a given quantity is called one at 180 degrees of air. This is 
simply a mathematical quantity, a convenient standard. It is called 1.0 because 
it is the limit of light passing through a lens having that supposed aperture. If 
that lens were used in water, the same quantity of light would be cut into a 
narrow angle by the reflective power of the water; so that, if you had 180 degrees 
in water, you would have a larger body of light passing through than you would 
have through 180 degrees in air. The whole difficulty of the opticians, I believe, 
at first was, not to get that, which is very easy, but to get a combination to take 
up those rays, and they have succeeded to some extent ; that is to say, they can 
make ordinary immersion lenses in water contain very nearly 40_per cent. more 
than the dry lenses take up. 
The CHAIRMAN: At our next meeting we shall not have time to carry on the 
discussion on this subject, supposing you were to make a motion to adjourn the 
discussion. The next meeting is a special meeting for special purposes. I am 
in your hands, if you think it is worth while to take up the question again at 
the next meeting but one. I think it is myself. Mr. Cooke thinks that, as the 
matter refers so much to Mr. Davis, he should have a chance of replying. That 
could be got if he will give notice of a paper to be read at the November 
meeting. It is quite certain that some of the remarks of Mr. Miles are a little 
hard upon Mr, Davis, and I am sure Mr. Miles would not wish to prevent him 
having a word to say on the subject ; and if he will give notice—which I think 
he will—to read a short paper at the November meeting, the subject can after 
that be closed. 
A member suggested that Mr. Davis should be asked to read a paper. 
The CHAIRMAN: I think we may leave it in that way. I think I can pro- 
mise that Mr. Davis will read a paper. 
Mr. STANLEY: I rise to move a vote of thanks to Mr. Miles for the trouble 
he has taken to make this matter clear to the younger members of the Society. 
We all know that it is an extremely difficult thing for young microscopists to 
know what kind of glass they should select, and Mr. Miles to-night has cer- 
tainly given us some idea as to what class of glass we should get. I think it 
all comes back to the original point, which is, that it entirely depends upon the 
class of work you are doing. For myself, I have got two, and the one most 
suited to my botany was one of very low power, and the price was low too. 
There is, however, one thing that this discussion has brought forward which I 
think is of real value to the Society, and that is the letter from Mr. Dallinger. 
I think that if we take that letter as the stand-point from which to judge any 
objective that we may require to get in connection with the work we are doing, 
we shall have something definite, and shall not make any mistake. As he 
clearly says in his letter, he of course uses different angular apertures for diffe- 
rent classes of work, and one wont do for the work of the other. It is therefore 
quite clear, and I think we are indebted to Mr. Miles for the great trouble 
which no doubt he has had in order to make himself acquainted with the vari- 
ous points in connection with objectives. 
Mr. MEsTAYER: I have pleasure in seconding the motion. 
The motion was then put and carried. 

