The London, Edinburgh and Dublin philosophical magazine and journal of science . he student of physical optics knows that this is so inactual fact. To study plane-wave phenomena, or to verifyplane-wave dimensional calculations, he must remove hissource of light, itself relatively small, to a considerable distancefrom his grating or other apparatus ; he must get his beamsof rays approximately parallel, that the normal wave-surfacemay be approximately plane. This necessity belongs to thenature of plane waves, 5. But considering now microscopic objectives, many suchhave been made as short in focu
The London, Edinburgh and Dublin philosophical magazine and journal of science . he student of physical optics knows that this is so inactual fact. To study plane-wave phenomena, or to verifyplane-wave dimensional calculations, he must remove hissource of light, itself relatively small, to a considerable distancefrom his grating or other apparatus ; he must get his beamsof rays approximately parallel, that the normal wave-surfacemay be approximately plane. This necessity belongs to thenature of plane waves, 5. But considering now microscopic objectives, many suchhave been made as short in focus as -^q of an inch. It isimpossible to regard light emitted from an object, as consistingof uniform plane waves on arriving at the surface of such alens, after a path of, perhaps, gj^ of an inch ; except in thecase of plane-ivave illumination of the object, as in the Abbetheory. Even in that case it would seem that considerablemodification must be made in Dr. Stoneys presentment ofevents in his fig. 1 (Nov. 1896, p. 433) here reproduced. His Tier. l. objective <% 1— Zya. beam Cb of uniform plane waves is represented as proceedingfrom the object C ; and supposing only plane waves to * Treatise on Physical Optics, p. 56 (1892). 484 Mr. L. Wright on Microscopic linages and Vision. illuminate the object, no objection need be made. But inthis case his first focus of the beam (described as parallel rays) at x, can no longer be regarded as a focus of raysemitted from the object C, but contrariwise, of rays pro-ceeding from the much more distant source of light. Andaccordingly, as a simple phenomenon, or fact of observation,it is not an image of any point of the object, but of the wholesource of light, which is focussed at #, and is seen there onremoving the eyepiece and looking down the tube. 6. Consider next the supposed dynamical system. This isby hypothesis set up, not by the object alone, or in ordinarymethod: We begin by positing repetitions of the objectivefield (Prop. 1,
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