THE MICROSCOPE AND ITS REVELATIONS. 



prism combination, Mr. Stephenson has devised a special form of sab- 

 otage Condenser (also made by Mr. Browning), which causes the illumi- 

 nating rays to issue from the object in two separate pencils, which will 

 strike the surfaces of the two prisms. This consists of two deep cylin- 

 drical lenses A and B, whose focal lengths are as 2.3 to 1, having their 

 curved faced opposed to each other, as shown in section at c; the larger 

 -and less convex being placed with its plane side downwards, so as to re- 

 ceive light from the mirror, or (which is preferable) direct from a lamp. 

 Tinder this combination slides a movable stop, with two circular open- 

 ings, as shown in Fig. 24. The lamp being placed in front of the instru- 



Double Stop for Stephenson Binocular. Stephenson's Erecting Prism. 



ment, the two apertures admit similar pencils of light from it; so that 

 each eye receives a completely equal illumination, and no confusion can 

 occur from the impinging of the rays on the lower edges of the prisms. 

 With this arrangement the Podura-markings are shown as figured by the 

 late Kichard Beck (Plate n., fig. 2); while the curvatures of the scale 

 come out with the distinctness peculiar to Binocular vision. 



36. But one of the greatest advantages attendant on Mr. Stephenson's 

 construction, is its capability of being combined with an erecting arrange- 

 ment; which renders it applicable to purposes for which the Wenham 

 Binocular cannot be conveniently used. By the interposition of a plane 

 silvered mirror, or (still better) of a reflecting prism (Fig. 25), 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 bodies, which are inclined at a convenient angle, as shown 

 in Fig. 26; so that the stage being horizontal the observer can look at 

 his object at the inclination which he finds most comfortable. The 

 .angle to which the prism is worked can be varied to suit individual 

 requirements; but if it should be desired to use the instrument with 

 Polarized light, it will be found advantageous that the reflection from the 

 surface A B should be at the polarizing angle of 56J, since, by substi- 

 tuting for the silvered mirror or prism a highly polished mirror of black 

 .glass, this will then act as an analyzer, with some decided advantages over 

 the Nicol prism, except in being incapable of rotation. The great value 



