DESCRIPTION OF PLATES
Plate I. (Frontispiece).
A. Radiant field consisting of white disk disposed on a black ground, with inset figure showing that a dark margin is developed round a glass spherule when placed in position against such a field. (Dark outline picture.)
B. Radiant field consisting of black disk disposed on a white ground, with inset figure showing that a bright margin is developed round a glass spherule when placed in position against such a field. (Dark ground illumination.)
C. Radiant field consisting of a homogeneous white surface, with inset figure showing that a glass spherule is only very lightly outlined when placed in position against such a field. (Suppression of outline picture. )
D. Radiant field consisting of a blue disk disposed upon a red field, with inset figure showing that a spherule placed in position against such a field is seen as a blue object delineated by a red outline (illumination by differentially coloured light).
E. Series of extemporized substage stops made in the manner described in Cap. II., Sect. II., subsect. 2., p. 20.
a. dark outline picture; b. bright outline picture; c. picture in relief; d. pure colour picture; e. colour and outline picture (vide pp. 3-4 and p. 12).
Plate II. (facing page 20).
a. Glass spherule, mounted in Canada balsam and illuminated by the fully open focussed condenser in such a manner as to obliterate the delimiting outline (vide p. 21, Exp. 3; p. 99, Exp. 1; and p. 174).
b. Glass spherule, delineated by a bright outline upon a dark ground. (Dark ground illumination.] The object, which is mounted in water, is illuminated by hollow, wide-angled beams furnished by a wide-angled condenser fitted with a central spot stop, as shown in Figs. 75 and 90, and is viewed with a narrow-angled objective.
c. Glass spherule delineated by a dark outline upon a bright ground. The object is mounted in water, is illuminated by narrow-angled beams and is viewed with a narrow-angled objective. (Dark outline picture.)
d. Film preparation of tubercle bacilli, differentially stained red, mixed with other bacteria which are stained blue. Preparations such as this here in question serve as a test for chromatic aberration in the microscope (vide p. 99, Exp. 2).
e. Reproduction of a drawing of a portion of a test diatom (triceratium). This serves to illustrate the difference between the "funda-
