STEPHENSON'S BINOCULAR IOI 



prism (fig. 77), above the tube containing the binocular prisms, 

 each half of the cone of rays is so deflected that its image is reversed 

 vertically, the rays entering the prism through the surface C B, being 

 reflected by the surface A B, so as to pass out again by the surface 

 A C in the direction of the dotted lines. Thus the right and the left 

 half-cones are directed respectively into the right and the left 

 I todies, which are inclined at a convenient angle, as shown in fig. 78 ; 

 so that the stage being horizontalthe instrument becomes a most 

 useful compound dissecting microscope, and as thus arranged by 

 Swift, with well adjusted rests for the hands, has but few equals for 

 the purposes of minute dissections and delicate mounting operations ; 

 indeed, the value of the erecting binocular consists in its applica- 

 bility to the picking out of very minute objects, such as Diatoms, 

 Polycystina, or Foraminifera, 

 and to the prosecution of 

 minute dissections, especially 

 when these have to be carried on 

 in fluid. No one who has only 

 thus worked monocularly can 

 appreciate the guidance derivable 

 from binocular vision when once 

 the habit of working with it has 

 been formed. 



Tolles's Binocular Eye-piece. 

 An ingenious eye-piece has been 

 constructed by Mr. Tolles (Boston, 

 U.S.A.), which, fitted into the 

 body of a monocular microscope, 

 converts it into an erecting stereo- FIG. 78. Stephenson's erecting 



scopic binocular. This conversion binocular (1870). 



is effected by the interposition 



of a system of prisms similar to that originally devised by MM. 

 Nachet, but made on a larger scale, between an 'erector' (re- 

 sembling that used in the eye-piece of a day-telescope) and a pair 

 of ordinary Huyghenian eye-pieces, the central or dividing prism 

 being placed at or near the plane of the secondary image formed by 

 the erector, while the two eye-pieces are placed immediately above 

 the two lateral prisms, and the combination thus making that 

 division in the pencils forming the secondary image which in the 

 Nachet binocular it makes in the pencils emerging from the objective. 

 As all the image-forming rays have to pass through the two surfaces 

 of four lenses and two prisms, besides sustaining two internal re- 

 flexions in the latter, it is surprising that Professor H. L. Smith, while 

 admitting a loss of light, should feel able to speak of the definition 

 of this instrument as not inferior to that of either the Wenham or 

 the Nachet binocular. It is obviously a great advantage that this 

 eye-piece can be used with any microscope and with objectives of 

 high power ; but as its effectiveness must depend upon extraordinary 

 accuracy of workmanship its cost must necessarily be great. 1 



1 See American Journal of Science, vol. xxxviii. (1864), p. Ill, and vol. xxxix. 

 (1865), p. 212; and Monthly Microsc. Journal, vol. vi. (1871), p, 45. 



