I4 THE NORTHERN MICROSCOPIST. 

operator to adjust his objective to any aperture he wishes, and this 
cannot be effected upon any older plan without having a large set 
of stops on hand. The closing of the shutter does not contract 
the absolute size of the field, but only the brightness of it, and the 
true value of Zene/ration can easily be observed without moving the 
eye from the tube. 
The value of wide apertures for good definition may also be seen 



when using this “shutter.” If Topping’s admirable preparation of 
the proboscis of the blow-fly be observed with an inch objective 
having an air angle of 30°, the view is superb, the pseudo-tracheal 
markings come out well-defined and sharp; but close the shutter 
until an angle of 14° or less is obtained, and examine again, when 
it will be found that the definition is not nearly so good, while 
there is more penetration, the whole of the pseudo-tracheal tube 
being observed under one focussing. While in this condition, the 
eye being still applied to the tube, open the shutter to its full ex- 
tent, and the effect of wide aperture will demonstrate itself. 
Perhaps the best object to show the amount of penetration pos- 
sessed by objectives of low angle, may be found in the micro- 
fungus Myxotrichum deeflxum, or M. chartarum observed under 
the one-inch objective. The former object consists of little patches 
of grey downy balls, from which arise a number of radiating threads 
furnished with a few opposite and deflexed branches. Under an 
inch objective of 30° air angle but few of these branches can be 
seen under one focussing, the remainder being enveloped in a 
haze of light ; but if a central layer be focussed, the simple closing 
of the shutter will suffice to bring the superior and inferior layers 
into view, though of course the image is not so bright or well-de- 
fined as before. 
The objection to wide angles, that they do not possess penetra- 
tion may now be fairly said to have broken down ; its other phase, 
that of working distance, we will treat of in a future number. 

