On Tolles Interior Illuminator. By William A. Rogers. 755 



to the flexure of the bars upon which the graduations are traced. 

 In this article he alludes briefly to the method of illumination 

 which he adopted, as follows : — 



" For central illumination of the divisions, small right-angular 

 glass prisms are affixed in the interior of the Microscope near the 

 objective, which are placed in the ends of short tubes and inserted 

 through lateral openings, reflecting the exterior light which passes 

 along the axis of the short tube vertically against the division, 

 being still more controlled by the objective. This interior illumina- 

 tion is, according to my experience, preferable to any other. It 

 produces sharp, well-defined images of the lines, and gives suffi- 

 cient light even when diffused daylight falls upon the face of the 

 prism." 



Immediately upon reading this description I went to BIr. Tolles 

 in order to obtain his assistance in the construction of an illumina- 

 tor of this form, being ignorant of the fact that he had as early as 

 1866 made one of exactly the same form. Inasmuch as at least 

 four persons seem to have independently suggested the use of a 

 prism inserted between the two lenses of the objective for the 

 purpose of securing illumination, it is well to insert here what I 

 believe to be the first published account of the invention. I quote 

 from the ' Annual ot Scientific Discovery ' for 1866-7, page 

 149:— 



" Mr. Charles Stodder exhibited before the Massachusetts 

 Institute of Technology, in December 1866, a new illuminator of 

 opaque microscopic objects under high powers, the objective being 

 its own condenser — the invention of Mr. Tolles. 



" The principal difficulty met with in passing a beam of light 

 down through the objective of a Microscope, and thus condensing a 

 strong hght upon an opaque object, is, in the case of high powers 

 especially, the reflection back of a considerable portion hy the 

 lenses of the objective. This causes fog and obscuration of the 

 image, though the object be well illuminated. This reflection 

 takes place principally at the interior front surface of the front 

 system. 



" To obviate this difficulty, a small rectangular prism, immedi- 

 ately above the front system, is so far introduced iuto the side of 

 the objective mounting as to slightly encroach upon the extreme 

 margin of the upper surface of the combination, "When the 

 parallel rays are reflected by this prism down through the 

 marginal parts of the front covered by it, they will have their 

 focus much beyond the place of the object. As a medium case tlie 

 distance of their convergence wodld be ten times the local distance 

 of the objective ; consequently a much greater portion of the whole 

 light incident upon the front system would lie transmitted, and 

 whatever amount experienced reflection would be dissipated liy 



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