THE ILLUMINATION 
OF MICROSCOPIC OBJECTS. 
By J. R. Jacx, M.I.N.A. 
(Read roth September, 1912.) 
ALTHOUGH most naturalists make considerable use of the 
microscope comparatively few obtain the best results which 
are possible. Considerable care is spent in focussing the 
instrument upon the object, but the observer is frequently 
puzzled to know why the same object can be seen very much 
better on some occasions than on others. In most cases this 
is due to variations in the illumination, and to obtain what 
is known as a “‘critical” image it is necessary to take as 
much care with the optical system on the condenser side as 
with that on the objective side. This may easily be seen 
by considering the principles underlying microscopic pro- 
jection. In fig. 1, let O be the object and L the object glass. 
I 
Fig. 1 
From every point in O rays diverge, some of which fall upon 
the object glass L, which brings them to a focus at the 
corresponding or “anti point” in the image. This image 
may either be received on a sensitive plate or may be further 
enlarged by means of an eye-piece. Since the image is 
formed only by rays which have followed such paths it is 
evident that no other light is of any value in forming the 
