228 



THE AMEKICAN MONTHLY 



[December, 



the objective "two truncated rec- 

 tangular prisms," which, as he de- 

 scribes and figures them, are not 

 only identical in form and function, 

 but are of almost precisely the 

 same size (compare Fig. 1, illus- 

 trating his paper with Fig. 33 in 

 this paper), as those of the second 

 instrument of Riddell. But the 

 American microscopist is not even 

 mentioned either in this or in his 

 second paper by Mr. Stephenson, 

 who explains that in the truncated 

 prisms thus used " the dispersion 

 caused by refraction at the first '' is 

 " corrected at the second trans- 

 mitting surface " without appar- 

 ently being aware that Riddell had 

 already pointed out this mutual 

 compensation of the two refractions 

 in the same arrangement of prisms 

 in his paper read at Cleveland in 

 July, 1853. 



Like Riddell, Mr. Stephenson 

 perceived that by the use of the 

 two truncated rectangular prisms 

 alone the images of objects seen 

 were laterally erected ; and like 

 Kiddell he proceeded to complete 

 the erection by the use of another 

 rectangular prism placed above the 

 first pair in such a position as to 

 effect a second lateral erection at 

 right angles to the first. I have 

 pointed out that Riddell attained 

 this object by placing a small rec- 

 tangular prism above each eye- 

 piece, and these prisms being mov- 

 able, the observer could look 

 through them at any desired angle, 

 although the tubes of the micros- 

 cope remained vertical. Mr. Step- 

 henson, however, placed his addi- 

 tional rectangular prisms immedi- 

 ately above the first pair, and made 

 it so large that a single prism an- 

 swered for both tubes ; at this point 

 the two tubes carrying the eye- 

 pieces were inserted into the up- 

 right tube containing the prisms at 

 an angle of 75°, which he thought 



the observer would find " with this 

 stand as convenient as any." 



This arrangement, however, was 

 of course less perfect than Riddell's 

 use of two rectangular prisms, one 

 for each tube, would have been, 

 and Mr. Stephenson soon perceived 

 this, for in a subsequent paper, read 

 to the Royal Microscopical Society, 

 March 6, 1872 {The Monthly Mi- 

 croscopical Journal, April 1, 1872, 

 p. 167), he described several im- 

 provements in his instrument, one 

 of which was that he cut his large 

 third prism in half, and cemented 

 the two pieces together " at such an 

 angle that the light enters and 

 emerges at right angles to the sur- 

 faces, instead of obliquely, as it 

 did in his first instrument; thus 

 making his arrangement optically 

 the same as Riddell's, except that 

 the upper rectangular prism of each 

 tube was below the eye-piece in- 

 stead of above it. 



He also modified the angle at 

 which his two tubes were inclined, 

 to the optical axis of the objective, 

 making it QQ^z" instead of 75°, and 

 he made the pair of reflecting 

 prisms next the objective very 

 much smaller than he had done be- 

 fore ; indeed " their dimensions are 

 so far so reduced that they are ca- 

 pable of being inserted into the ob- 

 ject-glass itself; this is accom- 

 plished by placing them in a small 

 brass tube, which is fixed in and 

 projects beyond the nozzle of the 

 instrument, but without in any 

 way affecting the screw." This 

 last alteration was undoubtedly a 

 real improvement on the original 

 instrument of Riddell, and greatly 

 benefits its perfonnance with the 

 one-foufth and one-eighth objec- 

 tives ; but it was after all mere- 

 ly carrying out Riddell's principle, 

 cited above, that in obtaining bino- 

 cular vision with the microscope, 

 the reflecting prisms should be 



