530 THE NEW IMMERSION ILLUMINATION. 
by Prof. Biscoe, by the common paraboloid alone, it being con- 
verted into an immersion instrument by filling its cup with water. 
These means of illumination, now for the first time available, 
may act in three distinct ways, one of which is new. 
The new method is applicable only to objects mounted dry, and 
is illustrated by Fig. 123. It has been called, by Rev. S. Leslie 
Brakey, Internal Illumination. All the light suffers total internal 
reflection from the upper surface of the slide except that which 
meets the surface at the points of contact of the object, and the 
rays thus excepted enter and illuminate the object. Of course the 
object must lie upon the slide, and beginners are often puzzled by A 
Fig. 126. 
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failure, not suspecting or rerhembering that the object may bare 
been mounted upon the under surface of the cover. a 
The two other methods are the common opaque illumination 
dark-field illumination, which are illustrated together in Fig 
as they are usually employed together in practice. 
applicable only to objects mounted in some medium, im Me 
the top of the cover becomes, optically, the first rosin 4 
by the light after entering the lens. The upper rey e 
represents this method of opaque illumination, the ray 
reflected by the cover upon the object. The lower Ye 
the dark-field effect, the object being illuminated por 
the common paraboloid, only the field is darkened net 
obliquity of the rays passing through the eover but ! 
that they are reflected back by it.* Hence its greater 
i anas, M 
* This illumination is not exactly represented in the a f the object, ad 
reflected down by the cover is that which passes by the aide © ut simplifie! Fig: 
which passes through the object. This is shown in Fig. 125, 
