go THE NORTHERN MICROSCOPIST. 

was, that an aperture beyond a certain limit injured the defining 
power of an objective, which was stated to be improved by cutting 
off some of the peripheral rays. 
This may be set forth more clearly by reference to Dr. Car- 
penter’s “ Microscope and its Revelations,” 6th Ed. “He (Dr. 
Piggott) has shown that the black margins, or black marginal 
annuli, of refracting spherules constantly displayed by small aper- 
ture objectives are attenuated gradually to invisibility as the 
apertures are widened to the utmost; that the black margins of 
cylinders, tubules, or semi-tubules also suffer similar obliterations, 
and that in consequence minute details are concealed or destroyed till 
the aperture ts considerably reduced.” 
Now, let us see what the Editors of the Journal R. M. S. say to 
this paragraph. ‘The statements that there is an inherent incom- 
patibility between good definition and large aperture should be 
deleted, as it is now known, both from theory and experiment, 
that the definition of objectives of the widest aperture (1-47 out of 
a possible 1-52) is as perfect as with those of less aperture. The 
wider the aperture of the objective the greater the technical skill 
required on the part of the practical optician ; but the notion that 
as the aperture of an objective is increased, its defining powers 
must necessarily, either on theoretical or practical grounds, be im- 
paired happily belongs to a closed chapter of microscopy.” In all 
these remarks there is no mention of the Aberrameter being used 
to avoid that bugbear of all antagonists to wide apertures—want of 
penetration. 
We have already on page 14 of this present volume figured the 
aperture shutter as made for us; it is very similar to the Aberra- 
meter, but as the aperture is not circular, Mr. Dancer has 
constructed the two forms shown in figs. 7 and 8. 
In fig. 7, the apertures are drilled in a sliding shutter, while in 
fig. 8 they appear as a wheel of diaphragms. The figures explain 
their use, and it will be seen that as the apertures are fixed, there is 
no gradation from one to the other. On one objective that we 
tried (a half-inch of 78°), the second aperture reduced the angle to 
56°,and the second to 36°, though of course the reduction must 
vary with each objective. Mr. Dancer has devised another method 
of using a limiting diaphragm, which may be found in a subsequent 
page. 

THE WORKING MICROSCOPE. 
OX ideas respecting the working microscope expressed in THE 
NORTHERN Microscopisr for September in last year have at 
last been carried out in Manchester. At the last Soiree of the 


