36 THE ILLUMINATION OF MICROSCOPIC OBJECTS, 
be shifted nearer the object the illuminant will have to be 
removed farther away, and a smaller cone of rays will fall 
upon the condenser, thus reducing the intensity of illumina- 
tion. If the condenser be moved closer to the illuminant a 
greater cone will be intercepted, but there will be a smaller 
angle of illumination at the object. These difficulties may 
be got over by using two condensers, the first, which is 
generally of the “‘bull’s eye” type, acting as a paralleliser, 
and having the illuminant at its principal focus. This lens 
projects a parallel beam along the optic axis. If the main 
condenser be now so placed that the object is in its principal 
focus the parallel rays proceeding from the paralleliser will 
focus exactly on the object, and critical illumination will be 
obtained. This arrangement can be set up by placing a light 
as far as possible from the condenser, and racking the con- 
denser up till the image just begins to lose sharpness; the 
condenser is then adjusted for parallel rays. The bull’s eye 
is adjusted to the illuminant so as to throw a sharp image of it 
on the farther wall, or it may be reflected up to the ceiling, 
and an iris diaphragm is used to reduce the size of the disc 
to that of the aperture in the principal condenser. The lamp 
and bull’s eye may then be put into the optic axis, taking 
care to keep their relative distance constant. As the rays 
between the bull’s eye and condenser are practically parallel 
the distance between these two is unimportant, and may be 
great enough to admit of various light filters being interposed. 
When this has been done correctly a transparent object is 
seen as if it were floating on a sea of light; when illumination 
is not critical it appears to be swimming 2m a luminous fog. 
When it is desired to photograph the image the question of 
colour in the object becomes important. 
The ordinary photographic plate is insensitive to red and 
very feebly sensitive to green light, while it is strongly 
sensitive to violet and invisible ultra violet light. With 
panchromatic plates the range of sensitiveness extends through 
the whole spectrum, and it is possible by using suitable filters 
to cut out certain parts of the spectrum and allow the photo- 
graph to be taken only by means of a selected narrow band. 
In the case of a subject faintly stained with violet or purple a 
photograph on an ordinary plate would be practically hopeless, 
because the light passing through the object would act as 
