

LETTERS TO THE EDITOR. 279 

copist who can smoke an ordinary cover-glass and bend a bit of tin, 
can just as easily dispense with. 
All who have bought objectives of high angular aperture have 
felt the want of penetration in observing opaque and other objects, 
such as Seeds, Pollen, Foraminifera, Polycistina, &c.; and to 
those whose pockets are limited, any instrument ought certainly to 
be welcomed which obviates the necessity of purchasing duplicate 
objectives of lower angular aperture. 
That the Davis Aperture Shutter fulfils this condition was well 
illustrated a few days ago upon a slide of Polycistina, placed 
under a half-inch objective of 80°, and dark-ground illumination 
with the condenser. The result was a glare,—no definition, no 
penetration ; but when the aperture shutter was applied, an 
exceedingly good dark-ground was obtained, with penetration 
sufficient to clearly define the whole of the interior markings of 
some of the larger cone-like forms. 
That the Shutter will prove a very useful addition to our list of 
accessories, admits I think of no doubt, and I have heard it 
favourably spoken of by more than one working microscopist. 
Yours respectfully, 
WILLIAM STANLEY, 

THE APERTURE SHUTTER. 
Sir, 
There has been a good deal of discussion of late about the 
merits of the new “Aperture Shutter”: perhaps you will allow me 
to give my testimony to its value both in photo-micrography and 
also in ordinary microscopic work. 
On exhibiting foraminifera, sand deposits, and big diatoms, I 
have frequently found that the power which gave most detail lacked 
sufficient penetration to bring all the objects on the slide into a 
common focus; a very slight increase of penetration would answer, 
and a lower power would not always give the desired effect. Since 
I have used the Shutter this difficulty has vanished ; for, by a suit- 
able contraction of the diaphragm, the desired structural detail is 
preserved, while all the objects are shown in good focus. With 
proper illumination the loss of light is inappreciable. Some may 
say, ““‘Why not use special low-angle lenses, which will give all 
requisite penetration?” Simply because penetration is far from 
being the only desirable quality in an objective. Lenses that possess 
great penetration usually possess little else, and the loss of light 
entailed by their use is far greater than that experienced when using 
a wide-angle lens with the Aperture Shutter. I have been a daily 
worker with the microscope for several years, and being advised to 
begin with low-angles, have been repenting ever since my having 

