Dr. Gr. J. Stoney on Microscopic Vision. 



433 



beam enters the objective at b, and by the objective is brought 

 to a focus at x, the focus for parallel rays. Here it forms an 

 Airy's spurious disk, or rather a somewhat more diffuse 

 spurious disk, since the objective falls short of being aplanatic 

 in its treatment of beams of parallel light incident on its 

 front surface. Past the focus x the beam advances, and 

 diverges as it advances. Let D be where the axial ray, the 

 dark line of the figure, intersects the optic axis of the micro- 

 scope. Note that the axial rays of all the other beams emitted 

 from the image C will, like this one, start from the point 

 where C is pierced by the optic axis of the microscope, and 

 will all intersect that axis again at D. At D an image of 

 C is formed on a somewhat curved surface that stands at 

 right angles to the optic axis of the microscope. Our 

 beam, diverging from x, when it reaches this surface con- 

 tributes its quota towards the formation of the image upon it. 

 After passing image D the divergent beam continues till 

 it reaches the front of the eyepiece. It proceeds through 



Fig. 1. 



the eyepiece and is by it brought to a second focus at y, 

 where the eyepiece can form an image of x. If we produce 

 the portion qy of the axial ray backwards, it will intersect 

 the optic axis at E, where the backward production of the 

 convergent beam qy will widen out so as to extend over 

 the space occupied by the virtual image E, and contribute 

 its share towards the formation of that image. Accordingly 

 the light of our beam, which advances in the direction qy, 

 will enter the eye (the pupil of which should be brought 

 close toy) in the same state as if it had come direct from the 

 whole extent of an image at E. After passing y the beam 

 again diverges, and by the optical action of the front half of 



