OPTICAL PRINCIPLES OF THE MICROSCOPE. 33 



of the Erecting Binocular consists in its TIG. 



applicability to the picking out of very 



minute objects, such as Diatoms, Polycyst- 



ina, or Foraminifera; and to the pro- 1 



sedition of minute dissections, especially 



when these have to be carried on in fluid. 



No one who has only thus worked monocu- 



larly, can appreciate the guidance derivable 



irom binocular vision, when once the habit 



of working with it has been formed. 



37. Tolled Binocular Eyepiece. An in- 

 genious Eye-piece has been constructed by 

 Mr. Tolles (Boston, U. S.), which, fitted 

 into the body of a Monocular Microscope, 

 converts it into an Erecting Stereoscopic 



Binocular. This conversion is effected by nson , s E Binocll]ar 



the interposition of a system of prisms 



similar to that originally "devised by MM. Nachet (Fig. 17), but made 

 on a larger scale, between an 'erector (resembling 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 

 reflections in the latter, it is surprising that Prof. 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. x 



38. NaclieVs Stereo-pseudoscopic Binocular. An ingenious modifi- 

 cation of Mr. Wenham's arrangement has been introduced by MM. 

 Nachet; which has the attribute altogether peculiar to itself, of giving to 

 the image either its true Stereoscopic projection, or a Pseudoscopic ' con- 

 version of relief,' at the will of the observer. This is accomplished by 

 the use of two prisms, one of them (Fig. 27, A) placed over the cone of 

 rays proceeding upwards from the objective, and the other (B) at the 

 base of the secondary or additional body, which is here placed on the 

 right (Fig. 28). The prism A has its upper and lower surfaces parallel; 

 one of its lateral faces is inclined at an angle of 45, whilst the other is 

 vertical. When this is placed in the position 1, so that its inclined surface 

 lies over the left half (I) of the cone of rays, these rays, entering the 

 prism perpendicularly (or nearly so) to its inferior plane surface, under- 



fo total reflection at its oblique face, and being thus turned into the 

 orizontal direction, emerge through the vertical surface at right angles 

 to it. They then enter the vertical face of the other prism B; and, after 

 suffering reflection within it, are transmitted upwards into the 



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. 



