On Angle of Aperture. By E. J. Slack. 239 
construction applied to an |th of some thirty degrees less aperture 
would not work still better, even upon surface markings, as it 
certainly would upon objects requiring much penetration. This 
suggestion is equivalent to asking whether the proportion of angle 
of aperture to focal length is such as to admit of the best cor- 
rections, hearing in mind Professor Abbe’s remarks. 
Zeiss’ Jth, not at all competing with the new Powell and Lealand 
glass in its specialty, shows how much more than has been 
expected can be done on the plan of a small angle and great 
working distance. This glass stands C and D eye-pieces well, and 
has a very remarkable amount of penetration, united to more than 
usual resolving powers. 
To sum up the results of experiments with various powers of 
different constructions : In the first place it appears that opticians 
have been encouraged to make excessive apertures substitutes for 
good corrections ; 2, that naturalists and physiologists have been 
too contented with feeble resolving powers, under the belief that 
any more capacity for resolution must mean less penetration : and 
they ought to demand more penetration than they have been 
accustomed to, and far greater resolving power in addition ; 3, 
that better illumination, and specially Mr. Wenham’s Eeflex Illu- 
minator, adds greatly to the resolving powers of really good small- 
angled glasses ; 4, that microscopists, and especially this Society, 
should so act as to secure opticians from the unfair treatment 
alluded to by Professor 'Abbe, and cause all the honour that is 
deserved to he awarded to those who will bring comparatively small- 
angled glasses to the highest degree of perfection in resolving as 
well as in penetrating power. 
