1880.] 



MICEOSCOPIAL JOUKNAL. 



223 



must be of sucli form, that the 

 faces, at which the hght is immer- 

 gent and emergent, shall form 

 equal angles with the face on 

 which the internal reflection oc- 

 curs " {J'OG. cit.), and with this ar- 

 rangement " to produce orthoscopic 

 binocular vision, simple, not erect- 

 ing eye-pieces are required." Ma_y 

 25, 1853, Prof. Riddell sent a copy 

 of the abstract just cited to the 

 Quarterly Journal of Microscopi- 

 cal Science, in which it was re- 

 printed (YoL I, 1853, p. 301). 



July 30, 1853, Prof. Riddell ex- 

 hibited this improved binocular to 

 the American Association for the 

 Advancement of Science, and made 

 a communication on the subject 

 which was published in the Pro- 

 ceedings of the Association (Yol. 

 YII, i853, p. 16), and also in the 

 JSFew Orleans Medical and Surgi- 

 cal Journal for November, 1853 

 (p. 321). In this communication 

 he began by describing and figuring 



the optical 



arrangement 



of his 



original binocular (Fig. 32, supra, 

 is copied from this paper). He 

 then pointed out that the pseudo- 

 scopic effects he had encountered, 

 when this arrangement was applied 

 to the compound microscope, were 

 avoided, if it was employed with- 

 out eye-pieces, and exhibited a 

 hinocular dissecting microscope 

 made in this way, remarking : " In 

 the smaller instrument before you, 

 this arrangement is observed. Used 

 without eye-pieces, it gives a stereo- 

 scopic and perfectly satisfactory 

 result. This instrument was con- 

 structed for a dissecting microscope. 

 I use it with lenses, whether plain, 

 doublets, or achromatics, from % 

 inch to 3 inches focal length. The 

 image is erect and orthoscopic." 

 But he adds : " If over B and B', 

 single oculars be placed, the bi- 

 nocular vision is found to be pseu- 

 doscopic ; that is, depressions ap- 



pear as elevations and elevations as 

 depressions. With erecting, or 

 double eye-pieces, analogous to those 

 of the terrestrial telescope, the 

 vision again becomes orthoscopic. 

 On this account, I prefer to reserve 

 this form of instrument for use 

 without eye-pieces, in the manner 

 described, and to construct the com- 

 pound binocular miscroscope on a 

 plan which I will soon explain." 



This new 



/d 



plan was repre- 

 sented in the 



paper 

 which 

 by a 

 which 

 nexed 

 (Fig 



from 



I cite 



cut of 



the an- 



iigure 



33) is a 



^2) > 



o 



Fig. 33. — Arrangement of 

 prisms abc 

 in Riddell's second plan 



copy. 



Immediate- 

 ly above the 

 objective (Z), 

 two prisms are 

 j)laced, on the 

 long side of 

 which total re- 

 flection takes 

 L place, the light 

 on each side 

 pursuing the 

 path indicated 

 on the right 



ova thefobjectives q{^q by the 



dotted line a, 

 h, c,d. " The light through the.objec- 

 tive which impinges upon a is, that 

 part of it which enters the prism, 

 refracted to the left, so that it 

 meets with the reflecting surface b. 

 Suffering total reflection it emerges 

 from the surface c, where from the 

 necessary identity of the immergent 

 and emergent angles, it is refracted 

 to the right, so as exactly to com- 

 pensate for its previous refraction 

 to the left." The equal angles, 

 formed by the short with the long 

 side of the prisms in this instrument, 

 were actually 45° ; the effect to be 



