16 PRINCIPLES OF MICROSCOPY
Again, the incident rays which come into consideration are represented as pencils of parallel rays. The component rays are designated on each side respectively by the letters a', b', c', d, c, b,a.
The rays d and d which pass through the centre of the sphere, and on either side the rays c and c', which traverse the central as distinguished from the peripheral regions of the sphere, fall outside the field of view of an observer who looks down upon the sphere. As a result, the central region of the sphere is dark in the retinal image.
The rays a', a which pass to one side and the other of the sphere, fall in the same manner out of the field of view, leaving the field of the image dark.
There are, however, as the diagram makes clear, other rays which enter the observer's pupil. There is on either side the refracted component of each of the rays b and b. This, as will be seen, is bent up in such a manner as to enter the pupil of the observer. There is further on either side the reflected component of the rays b' and b', which is bent upwards by reflection from the external surface of the sphere.
The refracted and reflected components of b and b', on either side, mutually aiding each other, give origin to the bright outline which delineates the spherule.
(c) Suppression of the outlines in the case of a highly refracting air-enveloped spherule illuminated by a combination of axial and oblique rays.
Figure 6 exhibits in a simplified form the conditions we have to deal with when an object is illuminated by a homogeneous illuminating field which we may for our purposes regard as sending out a centrally disposed pencil of axial and parallel rays and two beams of parallel and obliquely incident rays.
The blank image obtained under these conditions is, as has already been explained, the result of the imposition of the image produced by the oblique rays upon the image produced by the axial rays. The manner in which this is achieved will be realized by noting that in the diagram the components of the obliquely incident beams which come into consideration in connexion with the formation of the bright outlines are projected back by the observer into the marginal gap (a' to c') on the equatorial plane, which constitutes in the case where only axial illumination is employed the dark outline which envelopes the spherule.
