Highly Magnified Images. Bg J. W. Gordon. 13. 



therefore there is no improved delineation of detail. If you 

 diminish the scale of the image you diminish the scale of the 

 antipoint in the like ratio, and the details are therefore as sharply 

 defined in the small scale picture as in the large. One scale may 

 be more conveniently visible than the other, but intrinsically the 

 picture remains throughout all changes of scale self-identical. 

 This is the first great practical conclusion which Helmholtz has. 

 established, and not for the Microscope alone but for all optical 

 instruments which have aplanatic foci and flat fields. 



But it may be asked why then do not all Microscopes, equally 

 well corrected for spherical and chromatic aberrations, give equally 

 perfect images ? If scale has nothing to do with resolution, why 

 should an objective of wide angle possess higher resolving power 

 than a low powered objective ? 



The answer to that question is really very simple, but is not easy 

 to be expressed. Perhaps the clearest way to state it is to postu- 

 late that the object seen by the aid of any optical instrument is not 

 in strict truth the thing itself, but a simulacrum of the thing itself 

 produced by the instrument* If this seems to be an artificial view 

 of the case let it be considered that this is manifestly so when the 

 optical instrument is a coloured medium — say a piece of ruby glass.. 

 We know that in such a case the object seen is a profoundly modi- 

 fied presentment of the object as it exists. Less obviously, but 

 quite as truly, the appearance of an object seen through an aperture 

 is profoundly modified by the diffraction to which the aperture 

 gives rise. Every point upon the object is thereby converted into 

 an antipoint for presentation to the eye, and so in place of the 

 object as it is, delineated, as we may say, by points of light vary- 

 ing infinitely in colour and intensity, we have the object repro- 

 duced by means of antipoints, which not only vary in colour and 

 intensity, but which also encroach upon one another, and so blur 

 and complicate the whole result by their reciprocal interaction. 



"We are now in a position to answer the question, why does a 

 beam of wide angle yield a better image than that yielded by a 

 narrow angled beam ? The beam of light received by the aperture 

 of the instrument from any point upon the object may under this 

 point of view be regarded as an instrument — a pencil, say — by 

 which the supposed point is depicted in the optical field. If, now, 

 this pencil reproduces a point by a coarse antipoint, it will obviously 

 delineate a less perfect representation of the original than if it uses 

 a fine antipoint for that purpose. Now as the diameter of the anti- 

 point is inversely proportional to the sine of the divergence-angle 

 — 1/sin u — it is clear that the beam having the larger angle will, 

 ceteris paribus, yield the more exact picture. The difference is 



* This idea has been much insisted upon by Dr. Johnstone Stonev in several papers 

 on the theory of the Microscope. £ee the Phil. Mag., 5th ser., vol. klii., p. 426, et seq 



