THE NEW IMMERSION ILLUMINATION, 531 
ness and its applicability to large angular apertures. Its effect is 
superb with powers as high as J of 130° or 140°, especially when 
used with the binocular. 
In practice it is not easy, nor: often necessary, to separate 
the latter two methods. They separate themselves according to the 
character of the object. With an absolutely opaque object the 
opaque illumination will alone be accomplished, the dark-field effect 
(lower ray of Fig. 124) being necessarily suppressed ; and success 
will probably be difficult and only indifferently good. With suffi- 
ciently translucent objects the opaque effect would be insignificant, 
but the dark-field effect easy and. excellent. Objects just opaque 
enough to answer equally to both methods give a confused result, 
which might possibly be valuable in exceptional cases. 
The latest contrivance (Mr. Wenham’s, of course) for an immer- 
sion illuminator is a glass cylinder half an inch long, one side of 
which is ground off at an angle of 64° and polished to furnish 
an internally reflecting surface. The upper end approaches the 
bottom of the object slide, the interval being filled with water as 
in the use of an immersion lens, and the lower end is ground to 
à convex surface whose refracting effect on the pencil of light 
18 supplemented by a plano-convex lens placed below it. In fact 
we have something like a Wollaston’s doublet for a condenser, 
whose cone of light is twice bent by internal reflection so that its 
apex is in the position of the object between the cover and the 
slide. In the diagram (Fig. 126) only a central ray is repre- 
Sented ; but in use nearly all the light falling upon the lower lens 
4s brought to a focus on the object, giving an abundance of light 
and remarkable results with high powers. The apparatus is so 
Mounted on the sub-stage as to rotate around its own focal point 
aS a centre; and excels the former appliances in giving a more 
intense one-sided illumination, in confining the light to the object 
instead of lighting up everything in the neighborhood, and in 
allowing the slide to be moved or changed with facility. 
_Tnmersion achromatic condensers for transparent (bright-field) 
illumination have not yet received sufficiently extensive trial to 
 *Scertain their exact degree of usefulness; but they seem likely 
2 come into use as a means of increasing the available angular 
_Merture of immersion objectives, if not for other purposes. 
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