38 
JANUARY 23RD, 1854. 
THOMAS ROMNEY ROBINSON, D.D., PresipeEnt, 
in the Chair. 
On the recommendation of the Council it was Resolved, that 
the following By-Law be adopted :— 
«¢ That no Member whose subscription shall be due on the 
30th November in each year shall have the privilege of voting 
on or after that date, until his subscription be paid up.” 
The President read the following Paper on a new method 
of measuring the angular aperture of the objectives of micro- 
scopes. 
‘‘ Shortly after the commencement of those improvements 
which have made the microscope such a powerful instrument 
of research, it was observed that complete freedom from aber- 
ration and high magnifying power are not sufficient alone 
to give perfect vision to a certain class of objects; but that 
these essential qualities must be combined with a large angle 
of aperture. The scales of Lepidoptera and Thysanura, and 
still more the siliceous valves of certain Diatomacez, are well- 
known examples of this fact; and to the use of them as tests 
we mainly owe the astonishing progress which has lately been 
accomplished in this department of optical art. Several among 
us remember the admiration which was excited by the first 
objectives of 20° or 30° aperture, and which became still more 
enthusiastic when Ross carried this element to 60°, which was 
considered a ne plus ultra. But as objectives were improved, 
more difficult tests were found which stimulated progress, till 
angles of 170° and upwards have been obtained, by this great 
optician with us; by Nachet in France; and Spencer in Ame- 
rica. The combination of science and operative skill which is 
