Page:Principles of Microscope.djvu/41

Rh 5. Deductions from the above experiments.

We have learnt from the above—

(a) That where a spherical object possessing a high refractive index is illuminated from a light source of limited extent placed at some distance behind it on the axis of vision, we have in the retinal image an object delimited by a dark outline upon a bright field.

(b) That when the highly refractive object is illuminated from an enveloping radiant field, we have in the retinal image an object delimited by a bright outline upon a dark field.

(c) That where the spherule is illuminated both from a light source placed directly behind it and from a light source which radiates light upon it from every side, the retinal field is blank.

Consideration of Fig. 3 below will show that the blank picture obtained in Experiment 3 supra is a composite of the pictures obtained in Experiments 1 and 2. . 3. SERVING TO ILLUSTRATE THE FACT THAT A BLANK PICTURE IS OBTAINED BY THE IMPOSITION OF A "BRIGHT OUTLINE PICTURE" UPON A "DARK OUTLINE PICTURE."

The superposition of the bright central disc c of A upon the dark central disc c′ of B, gives us the bright central disc in the composite image of A and B.

The bright annulus b′ in B, superposed upon the dark annulus b in A, gives again a bright marginal annulus in the composite image of A and B.

Lastly, the bright field a in A, superposed upon the dark field a′ in B, gives us a bright field in the composite image.

The lessons learnt from the three first experiments are enforced by the lesson of Experiment 4. It is here brought clearly before the eye (a) that the light which radiates into the pupil from the central area of the sphere and from the surrounding field is light