A VERIFICATION DEPARTMENT. 255 
desired and certainly defined much better than the lower apertures, 
we made a change and have not regretted it since the day when 
the low angles were swept from the cabinet. Then came Professor 
Abbe’s paper, and our medium angles gave place to wider aper- 
tures, as by his teaching it appeared plainly that if penetrating 
power was required it could be easily obtained by placing a dia- 
phragm behind the back lens of the objective. 
Never having seen any idea or device for effecting the reduction 
of angle in this manner, we thought we had made a discovery, but 
on talking the matter over with Mr. Dancer, the well-known optician 
of Manchester, he produced a graduating diaphragm, made in 
1871 for this very purpose. His idea was to have an octagonal 
adjustable aperture (iris diaphragms were not then in vogue) with 
graduations on the edge figured at each 5, and an index so that 
the position of the diaphragm might be registered for each “ aper- 
ture.” After this we had one made in the form of an iris diaphragm 
and it works very satisfactorily. 
The question of working distance we have already disposed of, 
and we hope to have shown how resolution, definition and penetra- 
tion may be made to work in the same objective, so that it simply 
becomes a question of angle, and we agree to the statement on 
page 834 of the Journal already mentioned, “that neither penetra- 
ting objectives nor defining objectives are alone sufficient for all 
classes of microscopical investigation ;” for our own part we say— 
give us an objective of wide aperture and a graduating diaphragm 
as a means of reducing it when necessary. 
But there are apertures and afertures: it is one thing to pur- 
chase an objective of such and such an angle, and another to find 
that it possesses it when received. ‘The following episode will 
fully illustrate this :— 
“‘ At a meeting of several microscopists the conversation turned 
upon the motion of diatoms, notably that of the Navicule.” Said 
Mr. Wydangle, “It probably is caused by cilia, too transparent to 
be seen. No!” said Mr. Wurkhardt. ‘It is most probably caused 
by osmosis, by the escape of the fluids through some cavities in 
the frustules, and who knows but that the markings we see on JV. 
rhomboides may not be minute openings through which the fluid 
escapes.” Mr. Wydangle: “Well, to tell you the truth I’m rather 
sceptical about the markings on that shell; I have a Robinson’s 
eighth of 140° and have never been able to see any lines or mark- 
ings to say nothing of any appearance of holes.” ‘‘Well,” said Mr. 
Wurkhardt, “all I can say is that if your glass will not resolve Rhom- 
boides it must be badly corrected, or is not the angle you say”——The 
old tale “‘ Love me, love my dog”—Mr. Wydangle did not like to 
hear his objective run down, and declared it must be a good one, 
“for he had spent pounds with Mr. Robinson for years and he was 
