THE NEW IMMERSION ILLUMINATION. , 529 
glycerine or oil of cloves, to the under surface of the slide. It 
becomes, manifestly, an immersion spotted lens; though it has lost 
so much of its angular aperture, to say nothing of the difficulty of 
placing the object in its focus, that it can no longer be used as 
= ch, Its available action is that of enabling us to throw light 
obliquely into the slide at such an angle, ordinarily impracticable, 
that it shall-suffer total internal reflection from the top of the slide 
(Fig. 123), or from the top of the glass cover when that is opti- 
cally identified with the slide, as when we examine an object in 
balsam, glycerine, etc. (Fig. 124). 
For many years our best means of producing this effect was a 
prism, as shown in Fig. 125. A small prism is attached to the 
under surface of the slide, temporarily, by soft balsam or by oil 
or glycerine in the case of mounted specimens, or permanently, 
E 
y 
Fig. 125, 
oo 
n balsam be a blank slide which is to be used for the occasional 
of unmounted specimens. This arrangement gives 
oY little light, and so little control of the angles at which the 
; ight meets the axis of the object and the axis of the instrument, 
: that it has been but little used and with indifferent results. 
q Mr. Wenham’ s lens removes all these difficulties. It is easy to 
> a enough for moderately high powers; and the light can be 
| from n upon the object at a considerable range of angles and 
any side or from all sides at once. When light is to be sup- 
> a ftom one side only,it may be directed by a rectangnlar prism 
a a S prism, or an (inclined) achromatic condenser of long 
while jae small angle—such as a two-inch or three-inch objective ; 
: glass ™m all directions at once it is best supplied by the common 
— Sid. The latter effect ae be obtained, as explained 
T; VOL. VI. 
