254 THE NORTHERN MICROSCOPIST. 
The great majority of low-angle glasses in the market are under- 
stood, by those able to form an opinion upon the subject, to be 
made in not so careful a manner as those of wider angle—in fact 
they were introduced as a set-off against the cheap and imperfect 
foreign lenses, which found their way into England from the Con- 
tinent, and, though much superior to them, are zof thoroughly 
corrected. It is not to these objectives we have referred in the 
preceding paragraph, but to the best productions of the leading 
opticians. If any of our readers doubt this, let them try a half 
inch of 35° or 40° over a scale of Polyommatus argiolus, using a one- 
eighth inch solid ocular, or an F ocular magnifying 40, and then 
repeat the experiment with one of Messrs. Powell and Lealand’s of 
the same power, but of 82° airangle, or Spencer’s half-inch of 
100°, 
According to our experience, which has cost much “ siller” and 
not a few regrets, low angle objectives are useful but in one direc- 
tion. ‘They possess great working distance, and consequently can 
be used with ease ; they allow of the examination of preparations 
possessing considerable depth, and give plenty of room for dissect- 
ing purposes, or for the illumination of opaque objects. Beyond 
this property of working distance, low aperture objectives do not 
possess any advantage over those of wide angle; on the contrary, 
this latter class may be made to do all that the low angles will do, 
and a great deal more. It may be the fashion for microscopists of 
a certain school to ridicule that section of their brethren they are 
pleased to call Diatomaniacs, who work with wide apertures ; but 
there are an infinite number of problems which can only be solved 
by these glasses, and the application of a low angle to these would 
be of as much use as trying to discover the whole of Saturn’s moons 
with the unaided eye. 
Penetrating power, Professor Abbe tells us, stands in inverse 
ratio to the numerical aperture ; but this is only one of the elements 
which go to make up the depth of vision in the microscope. 
Now, if the microscopist requires working distance, it is abso- 
lutely necessary that he select objectives of low angle, but then he 
sacrifices resolving power and not a little definition. If he cares not 
for this attribute of working distance, nor wishes to see beetles 
crawling about the field of vision, or an occasional frog skipping 
around, then wide apertures will give him the most satisfaction in 
the long run, and will not cost so much as in buying low angles, 
and afterwards selling them at a ruinous loss. 
In our early microscopical days we believed implicitly in low 
angles ; we had persuaded ourselves that they were the best, and 
endeavoured to persuade our friends likewise. Thus matters con- 
tinued until we found by actual experience, free from any theoretical 
bias, that medium angles gave us quite as much penetration as we 
