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



at its centre. The luminous beams * of plane waves, each 

 emanating from the whole front of the surface of the object, 

 spread over this hemisphere, and the only case we need at 

 present consider is where the pupil of the eye (in naked-eye 

 observations) or the front lens of the objective (when we use 

 a microscope) takes in only the beams A, viz., those beams of 

 parallel waves thrown off from the surface of the object which 

 are directed towards the middle sector of the hemisphere, and 

 fails to admit the beams B, which arc directed towards the mar- 

 ginal parts of the hemisphere. The excluded beams are partly 

 Ba, those w T hich, if reversed, form the finer of the rulings that 

 go to build up the standard image. The rest of these beams, 

 viz. B6, are the moie oblique members of those fans of beams 

 which produce the coarser rulings — the whole of the standard 

 image being made up of these finer and coarser rulings (see 

 § 9), whereas the image seen by the observer is made up by the 

 beams A alone — by those which the front lens of the objective 

 can catch. 



Let us now define — B to be the same light as +B, except 

 that all the phases in — B are at each instant the reverse of 

 what they are in + B. In other words we get the light — B 

 by adding it to all phases in the light B ; hence if the light 

 -fB and the light — B are both present, they exactly cancel 

 one another. 



Now the whole light emitted by the object is A + B; and 

 it is this light which forms the standard image. Hence, if we 

 add the light — B to the standard image, and can find what 

 modification of that image is thereby effected, we thus arrive 

 at the best image which the light A can form : an image 

 wdiich the image actually formed on a large scale by the 

 objective may approach in perfection, but cannot exceed. We 

 may appropriately name it Standard Image No. 2. 



In order to arrive at standard image No. 2, we may add 

 the portions of light — Ba and — B5 in succession to standard 

 image No. 1, as these together make up the whole of the light 

 — B. The addition of — Ba simply obliterates the finer of 

 the rulings out of which the standard image is constructed. 



* It is convenient to use the word undulation where the waves extend 

 to an infinite or to an indefinite distance in their plane, and to employ the 

 terms beam and pencil where we intend the lateral extent of the waves 

 to be regarded as limited. 



Practically luminous beams of plane waves emanating from the 

 objective field, which is, of course, of limited extent, maybe used instead 

 of the undulations of the theory, which emanate from the entire objective 

 plane ; since the waves of a beam, unless very narrow, do not differ 

 sensibly from the waves of the undulation, except close up to its bounding 

 cylinder. 



