Theory of Illuminating Ai^'paratus. By Br. H. E. Fripp. 511 



For since, theoretically, any plane below the diaphragm (or even its 

 own aperture) may be considered the illuminating surface whether 

 the light reaches such a position directly or by means of reflecting 

 or refracting media, the best position of diaphragm may con- 

 sequently be affirmed to be ahove the last reflecting or refracting 

 surface, as thus the regulation of light-intensity may always be 

 controlled without any change of apparatus. But it is a matter of 

 daily experience that the position of the diaphragm in Microscopes 

 of various model, and in " condensers " of various construction, is 

 not dependent upon the principle contended for by Sir D. Brewster, 

 namely, of immediate proximity to the light-source, since in all 

 later constructions the diaphragm is a constituent part of the con- 

 densing apparatus, and situate always at some distance from mirror 

 or lamp. The reason for which may be found in the principles 

 here enunciated. 



What is, however, practically valuable in the illuminators of 

 Wollaston and Brewster, is not the application of a single narrow 

 axial pencil of light, but of a comparatively wide illuminating cone 

 whose base is formed by the radiant surface of the collecting lens 

 next the object. And the suggestion therein conveyed covers most 

 of the designs subsequently adopted of combining with an illu- 

 minating lens diaphragms pierced with apertures of various size 

 and form giving oblique, central, or full all round illumination. 

 But microscopy is more particularly indebted to Sir D. Brewster 

 for his emphatic advocacy of a scientific study of a too long 

 neglected subject. " The art of illuminating microscopic objects 

 is," he says, "of no less importance than that of preparing them 

 for observation," and he reiterates in the same article his con\dction 

 that " the progress of discovery with the Microscope must depend 

 upon the scientific illumination of the objects under examination." 

 And again, " I have no hesitation in saying that the apparatus for 

 illumination requires to be as perfect as the apparatus of vision ; 

 and on this account I would recommend that the illuminating lens 

 should be perfectly free from chromatic and spherical aberration, and 

 that the greatest care be taken to exclude all extraneous light, both 

 from the object and from the eye of the observer." * 



Dujardin and others taught the same doctrine that Wollaston 

 contended for, namely, that a converging light was best. Pritchard, 

 on the contrary, held that difficult test objects could be best 

 seen with diverging light. Harting, who mentions Pritchard's 



* The Jury in a Report on Microscopes exhibited at the first world's fair 

 (1851), mention Sir D. Brewster's method of illumination by achromatic lens 

 combinations placed beneath the stage, and state their belief that it is a great 

 advance on the simple condenser of Dr. Wollaston. But, as above shown by 

 diagr„mmatic construction, Dr. WoUaston's condenser effects the very same result 

 — u conjugate image and an illuminating pencil of considerable angular con- 

 vergence. 



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