THE ILLUMINATION OF MICROSCOPIC OBJECTS. 35 
image, indeed it may be seriously detrimental. In the case 
of objects illuminated by transmitted light,—and these are 
by far the most numerous,—the illumination should be such 
that the rays falling upon each point of the object shall 
arrive in a similar manner to that in which it is intended 
they shall depart on their voyage through the object glass. 
To achieve this result it will be necessary that the condenser 
C, and the source of illumination S, shall be arranged so that 
rays coming from any point in S shall focus at an anti-point 
in O. In fact the optical arrangement on the condenser side 
is precisely similar to that on the object glass side. It follows 
that an image of the source of illumination will be projected 
on the object by the condenser, and an image of this image 
will be projected by the object glass into the plane of the 
image of the object itself. This condition imposes certain 
restrictions on the nature of the source of illumination em- 
ployed. For low magnifications the flat side of a paraffin flame 
cannot be beaten, as it gives a plane surface of fairly uniform 
intensity. For higher powers the edge of the flame may be 
used. For the very highest the incandescent filament of a 
Nernst lamp gives good results, but for lower powers it would 
not be wide enough for its image in the plane I to cover the 
image of the object. If an incandescent gas mantle be used 
as the illuminant its image will be seen shining through the 
image of the object, and the same applies to an ordinary 
incandescent electric lamp. If either of these be used with 
ground glass in front the ground glass may be treated as the 
source of illumination, in which case the image of each facet 
of the ground glass will be projected into the plane I, and will 
show as a muddling of the surface. For visual examination, 
then, no illuminant is so satisfactory as the old - fashioned 
parafin lamp, In setting up the instrument a low-power 
objective should be focussed on the object, and the condenser 
racked up or down till the image of the lamp flame is 
clearly seen, when it is known that the flame is accurately 
focussed on the object. Any higher powered objective may 
then be used without disturbing the condenser system. It 
will be seen from the diagram that there is only one position 
of the illuminant for any given position of the condenser 
relative to the object, viz., that in which the illuminant is 
in the conjugate focus to the object. If the condenser 
