OBJECTIVES AND OCULARS 
9 
attainable by short wave-lengths in the application of ultra¬ 
violet light (X 25oo±) to photomicrography. In this way a 
resolving power of three times that obtainable with red light 
(X 75oo±) may theoretically be obtained. Since ordinary glass 
is practically opaque to rays below X 3000, it is essential that 
the condenser, objectives, oculars, object slides, etc., be made 
of quartz. For similar reasons quartz is preferable to glass in 
all ultramicroscopy, moreover, most glass exhibits a marked 
violet fluorescence under the influence of ultraviolet rays; quartz 
does not. 
SELECTING OBJECTIVES. 
It is evident from the above briefly outlined considerations 
that the choice of an objective of a given equivalent focus and 
magnification must depend upon the nature of the work the 
objective will be required to perform. In microchemical analy¬ 
sis, because of the rather unusual conditions which obtain, objec¬ 
tives must be selected with special reference to long working 
distance and great depth of focus; the brightness of field and the 
resolving power necessarily lost are, in this class of work, of 
little importance, since only low powers are employed and the 
indices of refraction of objects and surrounding medium are 
generally sufficiently different to permit an easy study of the 
preparations. When magnifications of from 300 to 500 are 
required in microchemical examinations, difficulty will be experi¬ 
enced in obtaining suitable objectives unless the prospective 
purchaser stipulates long working distances, since the working 
distance of those manufactured for the use of biologists is far 
too short to permit their application to the study of uncovered 
and therefore thick drops of liquid. 
For the study of objects lying in a single plane, for polished 
surfaces, rulings, fine etchings, etc., in which sharpness of out¬ 
line and delicacy of structure or tracery are present, flatness of 
field and high numerical aperture are essential. Our choice is, 
consequently, here restricted to aplanatics or to apochromatics, 
bearing in mind the fact that the resolving power of an immersion 
objective, where applicable, is greater than that of a dry one. 
