290 THE NORTHERN MICROSCOPIST. 

Now, from some cause or another,—perhaps my letter has not reached the 
gentleman,—I have, unfortunately, received no reply from Huxley. I am very 
glad that I received that letter from Mr. Dallinger, because, after all, he is our 
great authority. 
The CHAIRMAN invited discussion. He said: We have had the matter 
brought pretty fully before us. The opinion from Mr. Dallinger of itself is a 
very important one. I hope some gentleman will set the ball rolling. 
Mr. FILpDES: This is quite beyond me, but I should like to ask Mr. Miles 
if it occurred to him to ask Mr. Dallinger’s colleague in Liverpool, his opinion 
on this question, because, I presume, that gentleman is an authority on the 
microscope as well as Mr. Dallinger. His name is Drysdale. 
Dr. WEBBER: There is one thing I should like to point out. Iam not 
accustomed to use the lower powers, and cannot say anything at all about them. 
If you, however, take a blood corpuscle and magnify it 500 diameters, that 
would be nearly four millimeters in apparent size. The very highest aperture 
glass made, with no matter what eye-piece, can only divide that small space 
into something near 45 parts. I mean, if the glass were made as high as it 
possibly can be theoretically, there is not one glass made which comes even 
near to it. The ordinary 1-12th, of rather low angular aperture, will divide the 
same space into nearly 30 parts as far as I can remember—from 29 to 30, I 
think—and I don’t think there is such a very great difference between these 
two. You have 2-3rds or more in the dry cheap glass, than the low numerical 
aperture of the very best and dearest glass that is made. Another thing I wish 
to remark upon is that Mr. Davis speaks a good deal in his papers of Abbe’s 
test objectives, and how they work under high eye-pieces. Professor Abbé, in 
his paper, refers especially to what he calls empty amplification, pointing out 
that you can only divide within certain limits. 
Mr. NApPER: I have here one of the numbers of the Journal of the Micros- 
copical Society, and on the covers is a table of apertures. We are told that 180° 
of air aperture is equal to a numerical aperture of 1-o. On the same page we 
have objectives noticed of 180°, water immersion, which is equal to an aperture 
of 1.33 and 180° of homogeneous immersion, which is equal to an aperture of 
1.52. This seems to show to us how an aperture greater than r80® in air can 
be got. 180° in air would be equal to a numerical aperture of 1-0, and yet 
there is an objective noticed with an aperture of 1.52. 
Mr, MILEs: The first question was, I think, Did I think of writing to the 
gentleman who is Mr. Dallinger’s colleague? Well, I did not think of it at the 
time. I should have liked to have written if I had dared to take the liberty, 
and, as a matter of fact, I would have done, as this is an important question, but 
as I had written to three gentlemen, and as I had been very busy, I thought 
perhaps that would do. I thought possibly there would be those at this meet- 
ing who would be able to quote the opinions of various gentlemen, and that it 
would not, therefore, be necessary for me to get further opinions. I do not 
myself profess to be acquainted even with the names of gentlemen who are 
known in the microscopical world. There is another question which has been 
asked in relation to Swift’s ¢inch. I have one, and a most unworkable objective 
it is. I do not know what the aperture of that lens is, and I am quite unable 
to answer the question how it comes about that it works so close, because not 
knowing the aperture, of course we have nothing to guide us. It certainly 
works very close, and I believe the aperture is measured from the distance by 
which it works, and if that has anything to do with it, it must be a good deal 
higher than he puts it at. Mr. Napper has introduced a question that I am not 
well posted up in. It is as well to admit a thing at once. I cannot answer his 
question, but I think there is a gentleman present who can explain where the 
apparent difference exists, and if he will kindly undertake to do so, I shall take 
it as a favour. I have not been able to get hold of anything that explains this 
numerical aperture question. It differs somewhat, I know, from aperture illu- 
minations in air. Might I call upon Dr. Webber to explain? I think that 


