THE 
MONTHLY MICROSCOPICAL JOURNAL. 
MARCH 1, 1875. 
L— THE PRESIDENT’S ADDRESS. 
Delivered before the Royal Microscopical Society, February 3, 1875. 
Gentlemen, — In the last anniversary address a well-grounded 
hope was entertained and expressed that the unquestionable inten- 
tions of Mr. Layard, when First Commissioner of Works, &c., to 
afford to the Royal Microscopical Society some accommodation in 
the new buildings at Burlington House would ere long be carried 
out ; but this hope has hitherto been entirely disappointed, nor is 
there at the present time any prospect of its fulfilment. 
The war of the “ Angle of aperture ” has been waged during 
the past year with considerable vigour ; but it appears somewhat 
to be regretted that a quasi-judicial calmness and courtesy should 
not at all times have been manifested in the discussion of a purely 
scientific question. If in the discussion of a question of optics an 
alleged ignorance of some of the first principles of that science be 
manifested, and thereby a feeling of irritation rather than - one of 
compassion be evoked, surely the blame must rest more with 
individual temperament than with the nature of the question at 
issue. 
On this subject there are two propositions so self-evident that 
they may be taken as axiomatic : first, — no ray can contribute to 
the visibility of an object that does not proceed from the object, 
and reach the eye; and secondly — that no ray proceeding from 
an object under a microscope can contribute to its visibility unless 
it enters the object-glass: consequently all rays that, proceeding 
from the object, fail to enter the object-glass, as well as all rays 
entering the object-glass, that do not come from the object, are use- 
less so far as vision of the object is concerned, and may be entirely 
left out of consideration. Mr. Wenharn is unquestionably right in 
stating that if an isosceles triangle be described, the base of which 
is ten times the measured diameter of the face of the front lens, 
and the altitude ten times the measured distance of the focal point 
from the same surface, the vertical angle of that triangle will 
correctly represent the maximum available angle of aperture : it is 
admitted that stray rays may perhaps enter the object-glass at a 
much greater angle, little less perhaps than 180° ; but such are 
practically useless for any purpose of vision. 
VOL. XIII. 
i 
