THE MICROSCOPE AND ITS REVELATIONS. 



short tube, may be fitted either into a collar interposed between the 

 lower end of the body and the Objective, or into a cap placed over the 

 Eye-piece (Fig. 81, B), in the stead of the ordinary eye-piece cap. The 

 former arrangement, which is specially adapted for use with the Binocu- 

 lar Microscope, has the advantage of not limiting the field, but it stops a 



good deal of light; while 

 in. the latter, the image 

 is brighter, but a good 

 al^of the margin of the 

 'field is cut off. In the 

 Harley Binocular ( 68) 

 the analyzing prism is fit- 

 ted into a slide below the 

 Wenham prism, which is 

 drawn out when thepolari- 

 scope is not in use; while 

 in Swift's Challenge Bi- 

 &Sfigg$S "ocular, a similar slide is 

 fitted into the body above 

 the Wenham prism. In these arrangements, such advantage as is obtainable 

 by the rotation of the analyzing prism is of course foregone; and the same 

 sacrifice is made, when, in the Stephenson Binocular ( 36), the Iceland 

 .spar analyzer is replaced by a reflector. The Polarizing apparatus may be 

 worked in combination either with the Achromatic Condenser (by which 

 means it may be used with high power Objectives), or with either of the 

 ' black-ground ' Illuminators ( 104, 105), which show many objects 

 such as the horny polyparies of Zoophytes gorgeously projected in colors 

 upon a dark field. 



111. For bringing out certain effects of Color by the use of Polarized 

 Light (Chap, xxn.), it is desirable to interpose a plate of Selenite be- 

 tween the polarizer and the object; and it is advantageous that this 

 .should be made to revolve. A very convenient mode of effecting this, is 

 to mount the Selenite plate in a revolving collar, which fits into the 

 upper end of the tube that receives the Polarizing prism. In order to 

 obtain the greatest variety of coloration with different objects, films of 

 .Selenite of different thickness should be employed; and this may be ac- 

 complished by substituting one for another in the revolving collar. A 

 still greater variety may be obtained by mounting three films, which 

 separately give three different colors, in collars revolving in a frame re- 

 .sembling that in which hand-magnifiers are usually mounted; this frame 

 being fitted into the Sub-stage in such a manner, that either a single 

 Selenite, or any combination of two Selenites, or all three together, may 

 be brought into the optic axis above the polarizing prism (Fig. 82). As 

 many as thirteen different tints may thus be obtained. When the con- 

 struction of the Microscope does not readily admit of the connection of the 

 Selenite plate with the Polarizing prism, it is convenient to make use of 

 .a plate of brass (Fig. 83) somewhat larger than the glass slides in which 

 objects are ordinarily mounted, with a ledge near one edge for the slide 

 to rest against, and a large circular aperture into which a glass is fitted, 

 having a film of Selenite cemented to it; this ' Selenite stage ' or object- 

 oarrier being laid upon the Stage of the Microscope, the slide containing 

 the object is placed upon it; and, by an ingenious modification contrived 

 by Dr. Leeson, the ring into which the Selenite plate is fitted being made 

 movable, one plate may be substituted for another, whilst rotation may 



