(586 



SUMMARY OF CUEEENT EESEA.EOHES RELATING TO 



first prism is a second one, rather larger, but of similar form, the anterior 

 acute angle of which projects slightly over the axis. This prism also 

 diverts the other half of the rays to 7° from the axis. The pencils diverge 

 upwards at an angle of 14° and at about 8 inches above the apex of the 

 angle they are separated from each other by approximately the medium 

 distance of the eyes apart. The telescopes which replace the eye-pieces 

 have achromatic object-glasses and the usual double eye-lenses, and by 

 means of rackwork can be moved so far on their axes as may be requisite 

 for a decrease in the width of the eyes. The prisms are inclosed in a 

 box, which, like the tube of an ordinary Microscope, can be raised or 

 lowered by rackwork or a micrometer-screw. The size of the lower 

 prism must be regulated according to the largest objective-lenses em- 

 ployed ; the other may be somewhat (or even considerably) larger ; this 

 is advantageous in that a longer path of the rays in the glass moves the 

 apex of the diverging axes lower down, and thus aids in limiting the 

 height of the whole instrument. An aperture on the upper side of the 

 box, central with the principal axis, allows of the insertion of a tube 

 with a small opening, or an ordinary eye-piece, by means of which an 

 exact adjustment of the prisms is made, it being necessary that a point 

 of the object seen through the eye-piece in the centre of the field of 

 view after the prisms are removed should also be in the 

 Fig, 75. centre of the telescopic field when the prisms are used. 

 The question of illumination is an important point. 

 The plane-mirror is never sufficient alone for any kind 

 of binocular Microscope. The concave can be employed 

 if it is movable freely in the axis without lateral move- 

 ment. Still better is a plane-mirror in conjunction with 

 a movable convex lens, which can be removed from the 

 stage to double its focal distance. This mode of illumi- 

 nation is, however, very disadvantageous for the produc- 

 tion of sharp images ; the light is often far too dazzling, 

 and small diaphragms are consequently necessary. 

 "With the Stereo-Microscope these should never have the 

 apertures round, but in the form of slits, placed at right 

 angles to the edges of the prisms ; seven or eight of 

 these (from the narrowest to about 2 mm.) may be 

 arranged radially upon an ordinary wheel of diaphragms. 

 The plano-convex illuminating lens shown in fig. 75, 

 used with a plane mirror, is, however, a much more 

 suitable arrangement. Upon the plane surface of this 

 lens two prisms are so placed that their thicker sides 

 ^3;:2IJ!I> unite in the central line, this being parallel with the 

 edges of the prisms. Such a lens forms two separate 

 images of the illuminator ; the two sets of rays intersect 

 at a point a little above the lens, where the object is 

 to be placed. Both fields of view are in this way equally 

 illuminated with diffused light without showing an imago 

 of the illuminator itself. The lens has a radius of curvature of 32 mm. 

 and a diameter of 40 mm., and the angle of the prisms is 8^° ; for the 

 movement of the lens in the axis a space equal to twice the radius of 

 curvature is sufficient. It can also be made in a quadrangular oblong 

 form instead of round ; in this case a central piece half as wide as long 

 is sufficient ; the light has then freer access to the mirror. 



