38 PHOTO-MICROGRAPHS. 
This “spot lens” being properly focused, the 
object is illuminated by a cone of oblique rays 
emerging from the uncovered margin of the front 
lens of the condenser. Carpenter says of this: 
“Tt is one of the great advantages of this kind 
of illumination that, as the light radiates from 
each part of the object as its proper source, in- 
stead of merely passing through it from a more 
remote source, its different parts are seen much 
more in their normal relations to one another, 
and it acquires far more the aspect of solidity.” 
This difference in the appearance of a transparent 
object, according as it is illuminated by ordinary 
transmitted light or by black-ground illumination, 
is illustrated in Plate XI. In Fig. 1 of this plate 
we have the diatom Coscinodiscus oculis tridis illu- 
minated in the ordinary way, and in Fig. 2, the 
same diatom as seen by black-ground illumina- 
tion. The true form of the object is brought 
out by the latter method, and the appearance of 
solidity spoken of by Carpenter is well shown; 
while in Fig. 1 we seem to be looking at a flat 
disk, of the thickness of which we are unable to 
judge. 
Photographing by Reflected Light. 
| 
But little has been done in the way of making 
enlarged photographs of opaque objects by re- 
flected light. This is probably due to the fact 
that, for any except the lowest powers, there are 
1 «‘ The Microscope and its Revelations,’ sixth edition, p. 125. 
