180 The Exhibitor. By the Hon. and Rev. the Lord 8. G. Osborne. 
deal of study I have arrived at the conclusion four such apertures 
will meet all that can be desired. No. 1, a fine “slit,” in length 
about the diameter of the upper “drum,” i the centre. No. 2,a 
similar aperture, a little way, say not quite its own length, from 
the centre. Nos. 3 and 4, a pin-hole and triangle, also a little out 
of the centre; these may all be cut in one disk. 
On the stage I have two steel springs for holding the slides, 
giving the means of shifting them in any direction. 
. Now for the modus operandi. Having screwed the lower drum 
home to the extent of the screw, drop in the disk with small drum 
(I have three of different diameters and projection); on this place 
the aperture disk; place and cramp the Darker on the stage; turn 
the mirror, and place the lamp so as to get the aperture lighted up, 
but not with the brightness as with direct illumination ; put on a 
slide, say of Angulatum (I prefer those of “ Moller”); adjust the 
mirror until you get a rather red appearance on the slide at the side 
farthest from the lamp. Focus down on this: when you come on 
a bright clear light, like that obtained from ordinary sub-stage 
illumination, by moving the stage laterally you will arrive at a 
portion of the slide with darker and darker background as you con- 
tinue to move it. It will now depend upon manipulation of the 
mirror to bring the object on this ground to its best definition. A 
little practice soon makes this easy. Supposing you have got an 
“ Angulatum” well shown—i.e. on dark ground—clear and erisp ; 
turn the milled head of the Darker stage and you will thus 
intensify or moderate the light. Having found with any one 
aperture good definition of any one object in a slide, with your 
fingers you can bring any specimen in the slide over this spot. 
I have thus brought every single object in the Typen-Platte of 
Moller successively in view, developing their respective beauty in 
a way I had hardly thought possible. 
I rarely use a mirror, infinitely preferring the Abraham’s achro- 
matic prism adjusted to the mirror bar; I can by my hand so 
alter the direction and amount of light with this, 1 desire nothing 
better. 
I use the ordinary Bockett lamp and condenser, with a small 
flame edgeways to the prism. 
I have worked of late chiefly with a },th of Zeiss, which I find 
quite capable of resolving almost every test on which I have tried 
it: it brings out both sets of lines on the Rhomboides, showing 
where they intersect. A 4 inch of Andrew Ross also gives me 
wonderful definition. The 4 and 1 inch of the same maker, with 
binocular, the objects on pitch-black ground, bring out many with a 
distinctness I have failed to get any other way. 
I prefer objects mounted dry, but seldom fail with any; asa 
proof of the way I now get definition; having dissected and 
mounted the gizzards of the Rotifers, 1 thought I knew their 
