434 Dr. Gr. J. Stoney on Microscopic Vision. 



the eye this divergence is lessened and at the same time the 

 axial ray is bent in so as a third time to intersect the optic 

 axis of the microscope, where that axis prolonged pierces the 

 retina of the observer. Upon the retina, round this point, 

 the beam of lessened divergence spreads and there assists in 

 the formation of image F, the image of the microscopic object 

 which is formed by the microscope upon the retina of the 

 observer. 



Twice upon its journey the beam we are dealing with has 

 been brought to a focus — at x and at y. At both these posi- 

 tions it is concentrated into a point or, to use more accurate 

 language, into a spurious disk, and can be seen as such. It may 

 be seen at x by removing the eyepiece and looking down the 

 tube of the microscope, taking care to keep the eye central ; 

 and it may also be seen at y by replacing the eyepiece and 

 looking with a magnifier at the image y formed in the air 

 outside the eyepiece *. 



Accordingly every visible point of the image seen at x 

 when we look down the tube of the microscope is the con- 

 centrated light of one of the beams of uniform plane waves 

 which has been emitted from the whole extent of the objective 

 field. We may therefore, by scrutinizing the position and 

 brightness of the points of this image, learn in what directions 

 and with what intensity the beams of uniform plane waves are 

 being thrown off from the whole front of the objective 

 field. 



We must not, however, suppose that by scrutinizing this 

 image we can learn everything about those beams, inasmuch 

 as they differ from one another not only in those respects of 

 which the eye can take cognizance, but also in the form and 

 position of the elliptic motion in each wave front, and in its 

 phase at a given instant — particulars which, though the eye 

 cannot detect them, are of primary importance in determining 

 what image of the microscopic object they will contribute to 

 form when they reach the position D. 



To complete this survey, we should trace the course of the 

 light up to the microscopic object, as well as from that 

 position forward. The light as it reaches the objective field 

 would be ideally perfect if it could be resolved into beams of 

 plane waves, perfectly uniform, coming with equal intensity 



* The effect is most striking when the object is such as directs into a 

 few definite beams each beam of the light supplied to it by the condenser. 

 An excellent object is the diatom known as Peristephania eutycha, especially 

 when the iris diaphragm is nearly closed, so that only a small sheaf 

 of beams, each extending over the whole objective field, is supplied by 

 the condenser. 



