ACCESSORY APPARATUS. Ill 



for the purpose of moderating the glare of the flame or of correcting its 

 yellowness; but as the chimney cannot be conveniently changed when- 

 ever the full light is required, the Author much prefers making such 

 1 light-modifiers' a part of the Illuminating apparatus attached to the 

 Microscope itself: and this may be done in different modes, according to 

 the construction of the instrument. Thus, when the Webster Condenser 

 ( 100) is in use, it may be furnished with three caps made to slide upon 

 its upper portion; one of them fitted with a disk of blue-glass, second 

 with one of neutral-tint glass, and the third with a finely-ground glass. 

 And in Swift's Combination Sub-stago ( 112) similar disks maybe made 

 to drop into the openings of the rotating plate; so that one may readily 

 be changed for another, or, if all three be placed in the plate at once, an 

 object may be examined under any one of them by merely rotating the 

 plate. Every ordinary Diaphragm-plate ( 98) ought to have its largest 

 aperture fitted, by means of a projecting shoulder, to carry such a set of 

 disks. The three arms on which the rotating Selenites are attached to 

 the Sub-stage of Messrs. Beck's First-class Microscope (Fig. 82), may be 

 fitted with similar disks, each of which may then be used either sepa- 

 rately or in combination with one or both of the others. Every ' Light- 

 modifier ' -should be so constructed and worked, that the light should be 

 made as nearly as possible to resemble that of a bright white cloud. For 

 this purpose a white-cloud Reflector may be easily made either flat, by 

 casting a Plaster of Paris disk upon the plane surface of the mirror or 

 concave, by casting it on the surface of a glass globe; the light reflected 

 from the surface of the plaster requiring to be condensed for the illum- 

 ination of small objects. Very pleasant white-cloud effects may be- 

 obtained by methods adppted by Mr. Slack. For large objects, viewed 

 with powers of 1 to 4 inches, he places under the stage a tube holding 

 a large disk (1J- inch diameter) of ground glass, the ground surface being 

 protected by a plain glass cover over it. By this means the peculiar tint 

 of the freshly ground surface is permanently retained. For 2-3ds and 

 half-inch powers he employs a glass slide carrying a disk or square of 

 thin paper, saturated with spermaceti, and protected from dirt by a thin 

 glass cover that adheres to it. This slide, disk downwards, is placed 

 under the object. Under still higher powers, some objects may be very 

 conveniently illuminated by a small bull's-eye finely ground on its flat 

 surface, and fixed with its convex face downwards in a tube that slides 

 into the Sub-stage fitting. 



110. Polarizing Apparatus. In order to examine transparent objects 

 t>y Polarized Light, it is necessary to employ some means of polarizing 

 the rays before they pass through the object, and to apply to them, in 

 some part of their course between the object and the eye, an analyzing 

 medium. These two requirements may be provided for in different 

 modes. The polarizer may be either a bundle of plates of thin glass, 

 used in place of the mirror, and polarizing the rays by reflection; or it 

 may be a ' single image' or 'Nicol' prism of Iceland Spar, which is so 

 constructed as to transmit only one of the two rays into which a beam of 

 ordinary light is made to divaricate by passing through this substance. 

 Of these two methods, the ' Nicol ' prism is the one generally preferred, 

 the objection to the reflecting polarizer being that it cannot be made to- 

 rotate. This polarizing prism is usually fixed in a tube (Fig. 81, A, #), 

 furnished with a large milled-head, c, at the bottom, by which it is made 

 to rotate in a collar, Z>, that screws into the Sub-stage fitting. For the 

 analyzer a second 'Nicol' prism is usually employed; and this, fixed in a 



