PHOTOGRAPHING BY REFLECTED LIGHT. 87 
XII. PHOTOGRAPHING BY REFLECTED LIGHT. 
Iy the section on Light, some remarks have been 
made on the subject of photographing opaque 
objects by reflected light. There are serious diffi- 
culties to be encountered, which interfere with 
the use of high powers for this purpose; but 
with comparatively low powers many interesting 
microscopic objects may be photographed which 
it would be quite impracticable to photograph by 
transmitted light. The light is thrown upon the 
object by means of a Lieberkiihn, and the writer 
has found the best source of illumination to be 
sunlight reflected from the sky. Direct sunlight 
reflected from the mirror of a heliostat has not 
proved satisfactory. 
Means must be provided for focusing the Lie- 
berkiihn separately from the objective, so that 
the best possible illumination of the object may 
be obtained. The writer’s method of accomplish- 
ing this has been described in Section II. For this 
kind of work it is essential that the objective have 
the greatest possible penetrating power. 
No light must enter the objective except that 
reflected from the object; and to secure good defi- 
nition it will be necessary to use a diaphragm 
between the objective and the object. A very 
long exposure will be required. The head of a 
fly, magnified about twenty diameters — three- 
inch objective — required an exposure of twenty- 
five minutes. 
