224 



THE AMERICAN MONTHLY 



[December, 



expected from other angles was dis- 

 cussed in the paper. The instru- 

 ment was an upright one, and the 

 two tubes were adjustable both at 

 top and bottom, " so that their in- 

 clination to each other may be 

 varied ; and the whole arrangement 

 slides at pleasure, horizontally, in 

 order to adapt the distance to the 

 eyes of different observers." Ar- 

 rangements for adjusting the an- 

 gle of the prisms to each other in 

 accordance with the varying posi- 

 tion of the two tubes were also pro- 

 vided. 



In this instrument, as the in- 

 ventor pointed out, "orthoscopic 

 vision is produced by the ordinary 

 single oculars." As to the mode of 

 illuminating the objects examined, 

 he observes : " Opaque objects may 

 be illuminated by the bull's-eye 

 condenser ; and transparent objects 

 by two concave mirrors, aided by 

 two diaphragms or screens, or one 

 large ' concave mirror and two 

 screens. At night, two candles may 

 be used conveniently with one mir- 

 ror. To illuminate for the higher 

 powers, a single achromatic con- 

 denser suffices.' " As the instru- 

 ment was incapable of being in- 

 clined, two adjustable rectangular 

 prisms were provided to fit over the 

 eye-pieces, by means of which the 

 image could be " viewed at any in- 

 clination between vertically and 

 horizontally, which may be con- 

 venient to the observer," But 

 these prisms had a further effect 

 which Riddell did not overlook. 

 He had observed that, although his 

 new arrangement, used with ordin- 

 ary eye-pieces, gave orthoscopic 

 vision in the sense that the natural 

 elevations and depressions in objects 

 examined truly appeared as such, 

 yet the image was erected in one 

 plane, while in the other it was not ; 

 and he pointed out that the erec- 

 tion would be completed by the use 



of the rectangular prisms above the 

 eye-pieces : " It will be seen," he 

 writes, " that the prism at A " (one 

 of the two just above the objective), 

 " has the effect of erecting the im- 

 age in one plane; while the small 

 prism at Z> "(above the correspond- 

 ing eye-piece), " can be placed so 

 as to erect it in the plane precisely 

 transverse. Thus the movements 

 upon the stage will be seen through 

 the instrument to be natural or 

 erect." 



The character of the binocular 

 vision obtained by this instrument 

 was described by Riddell in glow- 

 ing language. As the observer 

 gazes, the objects examined, "seem- 

 ingly hung in mid air, stand out in 

 all the boldness and perfection of 

 relief, and definiteness of position 

 in width and depth, which he has 

 been accustomed to realize without 

 glasses in the natural objects around 

 him." 



Riddell sent a copy of this re- 

 markable communication to the 

 Quarterly Journal of Microscopi- 

 cal Science, by which it was re- 

 printed in the number issued in 

 January, 1854 (Yol. II, p. 18). 

 Immediately after reading his paper 

 to the American Association for the 

 Advancement of Science, he re- 

 quested the Grunow Brothers to 

 construct for him a more finished 

 instrument on the plan of the work- 

 ing model of the improved binocu- 

 lar compound microscope then ex- 

 hibited and described above. Mr. 

 Wm. Grunow of New York, in 

 reply to an inquiry on the subject, 

 wrote me. May 20, 1879, that this 

 request was made of the Grunow 

 Brothers, then in New Haven, 

 Connecticut, in August, 1853, and 

 that the instrument was finished 

 and sent to him in March, 1854. 

 This is the identical instrument 

 presented by Mrs. Riddell to the 

 Army Medical Museum. 



