PROFESSOR MARTIN DUNCAN’S ADDRESS, 307 
‘Taking for our wait the capacity of an objective for collecting 
the whole hemisphere of rays from an object in air (z.e, the case of 
a dry objective of 180° angle) we obtain the “ numerical ” notation, 
which commencing with the lowest numbers advances as far as 1.52 
with oil-immersion objectives, and by the use of which not only 
are apertures compared in the same medium, but in different media 
also, and we see whether they are smaller or larger than the maxi- 
mum of a dry objective. 
It is gratifying to find that the reproach hitherto attaching to 
microscopists, for the use of misleading notation, is, thanks to the 
efforts of this Society, being rapidly removed, and that the initials 
N.A. are no longer so mystic a symbol as they have been. I under- 
stand that many of the opticians have decided to use the numerical 
notation in the future issues of their catalogues, which is a step in 
the right direction, which we shall hope to see generally followed. 
Whilst on this subject I may point out how important it is that 
in observations with high-power objectives, their aperture as well as 
magnifying power should be stated. Whether a large or a small 
aperture has been used, may make a very material difference in the 
value to be attached to the results described. 
The “ Homogeneous Immersion” prinaple. - 
The utility of homogeneous-immersion objectives being estab- 
lished beyond doubt by practical experience, it is interesting to 
note that the origin of the principle is very fully recognized by 
Professor Abbe to be due to our esteemed Fellow Mr. J. W. 
Stephenson. 
The two essential points in homogeneous immersion are, rst, 
the increase in aperture obtained by the use of a fluid of high 
refractive index and, 2nd, the enhanced optical performance arising 
from the total suppression of spherical aberration in front of the 
objective. Professor Abbe states that although Amici first applied 
oil immersion, he failed to recognize the specific advantage of an 
immersion fluid being as near as possible in refractive and dis- 
persive powers to the crown glass (Ze. “ homogeneous”). He 
finished his lenses and then sought for oils and mixtures of oils of - 
various refractive powers for obtaining the best correction. “It 
was Mr. Stephenson who, in his first communications with me, 
expressed the opinion that doing away with the anterior aberration 
would improve the defining power, and especially would afford very 
favourable conditions for further increase of aperture.” 
The importance of this system will be appreciated when we 
remember, in regard to the first point (the increase of aperture), that 
the theoretical resolving power of an objective is thereby raised 
from 96,400 lines to an inch, which is the maximum of a dry 
«SAEs 
