Page:Principles of Microscope.djvu/42

14 whose source is to be found on the visual axis directly behind the object, and (b) that the light which radiates into the pupil from the marginal area of the sphere is obliquely incident light derived from the outlying regions of the illuminating field.

6. Manner in which outlines are produced and suppressed.

We pass to consider why the radiant points which constitute the object picture arrange themselves in Experiment 1 in such a manner as to leave a dark annulus round the margin of the object; in Experiment 2 in such a manner as to furnish a bright annulus; in Experiment 3 so as to furnish a blank picture, and in Experiment 4 in such a manner as to furnish a red outline on a blue field.

We shall most conveniently deal with this problem by conceiving of the light projected upon the spherule as consisting in each case of a system of parallel rays, derived, as the case may be,

. 4. SHOWING THE MANNER IN WHICH THE "DARK OUTLINE PICTURE" IS PRODUCED.

from a light source disposed immediately behind, or as the case may be excentrically behind, the object.

(a) Formation of dark outlines in the case of a highly refracting air-enveloped spherule illuminated by central white disc disposed on a dark field.

Figure 4 illustrates the formation of dark outlines in the case