306 THE NORTHERN MICROSCOPIST. 

that it was compelled to carry the subimaginal skin poised above 
its body at the tip of the damaged wing for some hours, presenting 
the most curious appearance of carrying its own exquisitely made 
garment, until I summoned the aid of chloroform and detached it. 
One specimen was unable to extricate itself from its nymphal skin, 
being drowned in the attempt. This I have preserved in my 
cabinet in the act of escaping. 
This species is to be found from July to September emerging 
from cool streams to assume its final forms of short-lived aerial 
beauty. 
EXTRACT FROM 
PROF. MARTIN DUNCAN’S ADDRESS 
To THE MEMBERS OF THE ROYAL MICROSCOPICAL SOCIETY, 
FEBRUARY 8TH, 1882, 
The Abbe Theory of Microscopical Vision. 
( Continued from page 278. ) 
S Fellows of this Society we may, I think, be proud of the able 
communications, relating to this subject, which were published 
last year in the April and June numbers of the Journal. 
Numerical Aperture. 
The abandonment of the angular notation for aperture neces- 
sarily follows, as soon as the correct view of aperture is appreciated; 
for when we know that the apertures of three objectives are, for 
instance, as 98, 126, and 138, no one would insist that they should 
be designated 157°, 142°, and 130%. A notation can have no title 
to be considered a scientific one, which denotes things as the same 
when they are really different (60° in air and oil) or different when 
they are the same (180° in air and 82° in oil). 
Until, however, the “law of aplanatic convergence” had been 
demonstrated by Professor Abbe, no principle had been established 
by which the ratio between emergent beam and focal length, could 
be conveniently denoted. 
It would not be possible for me to condense, without a sacrifice 
of intelligibility, the steps by which he subsequently showed, in a 
very beautiful manner, that the ratio in question can be expressed 
by the product of the refractive index of the medium in front of 
the objective, and the sine of half the angle of aperture, that is by 
nm sin u, 


