Page:Principles of Microscope.djvu/43

Rh of an air-enveloped spherule upon which is projected a pencil of rays running parallel to the axis of vision.

In the optical section which is here in question four sets of rays lettered a, b, c and d respectively come into consideration. The rays a, passing as they do wide of the glass sphere, give origin to a system of points of a′ a′ which radiating into the eye produce the bright object field against which the dark outline of the spherule is defined.

The rays b and c are in part reflected away from the outer surface of the spherule, in part they pass into the spherule undergoing refraction as they enter. The refracted component of c along with the axial ray d, which passes through the spherule undeflected, are

. 5.

SHOWING THE MANNER IN WHICH THE "BRIGHT OUTLINE PICTURE" IS PRODUCED.

received into the eye forming the bright central area of the spherule. It will be seen that the gap a′ to c′—c′ being the point upon the equator upon which the ray c is projected from the eye of the observer corresponds to the dark marginal area which delimits the spherule.

(b) Formation of bright outlines in the case of a highly refracting air-enveloped spherule illuminated by a bright field surrounding a central dark spot.

Figure 5 illustrates the manner in which bright outlines are produced in the case of a glass sphere, enveloped in air, and illuminated by a corona of obliquely incident light such as is projected upon the sphere when placed in position against a black central disc mounted on a white field.