96 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



altliough tlieir inclination to the axis is different, leave tlie objective 

 as pencils of parallel rays, and therefore of themselves produce no 

 image, or rather one at an infinite distance. The convergence of the 

 pencils of rays requisite to produce a real image is effected after- 

 vpards by means of the eye-pieces, which consequently it would be 

 more correct to regard as telescopes, though they consist, like ordinary 

 microscopical eye-pieces, only of two plano-convex lenses of crown 

 glass, the ratio between their focal lengths being about 1:3. It will 

 be seen at once that, by employing this telescopic eye-jjiece to receive 

 the pencils of rays emerging parallel from the objective and coming 

 as it were from an infinite distance, it is not necessary that Micro- 

 scopes thus constructed should be of a fixed length. The length may 

 be altered at will without producing any change in the amplification 

 and distinctness of the image after it has been once obtained, provided 

 the telescopic ej-e-piece is so adjusted, by means of a draw-tube 

 arrangement, that distant objects can be clearly seen by it. It is 

 equally obvious how, by this process, the exact parallelism of the 

 l)encils of rays emerging from the objective, and consequently the 

 position of the object in the focus, is regulated and known. This 

 furnishes us with a basis which renders it possible to obtain such a 

 direction for each half of the pencil of rays by a single reflection that 

 each eye can take in one of the halves. 



In the original axis of the Microscope there are placed two glass 

 prisms, a smaller, A, Fig. 4, and a larger one B, which are fixed in 



such a manner that the smaller prism 

 causes one half of the rays and the larger 

 prism the other half to be diverted from 

 the axis under different angles by total 

 reflection. The two pencils D E of 

 j)arallel rays, are directed into the eye- 

 pieces through two tubes which converge 

 slightly towards the lower extremity. 

 The original axis of the Microscope lies 

 horizontally, and on the right of the 

 observer is the objective C, the stage, 

 and the illuminating apparatus ; the 

 observer looks do^-n from above (in a 

 direction inclined as may be desired) 

 through the two converging tubes, 

 directly upon the horizontal axis and 

 with each eye over one of the two reflect- 

 ing prisms. The first of these of course 

 projects only as far as the axis, so as to 

 leave half the opening free for the second. 

 They are so arranged on the axis that 

 they, with the eye-pieces to which they 

 are attached, can be moved by rack and 

 pinion so that their distance apart corresponds with the distance 

 between the eyes of the observer, without the image being affected 

 by the difference or alteration in the course traversed by the pencils 



Fig. 4. 



