182 



THE MICROSCOPE. 



combination A is computed to be used as an effective 

 spot-lens from a 3-inch objective up to a sixth, c C are 

 two small milled heads by means of which the optical 

 combination A is centred to the axis of the objective. 

 The revolving diaphragm E has four apertures for the 

 purpose of receiving central stops, oblique light discs, 

 and selenite films. D is a frame carrying two revolv- 

 ing cells, into one of which a mica film is placed, which 

 can be revolved with ease over either of the selenites 

 below, whereby changes of colour can be obtained in 

 experimenting with polarised light. The darts and 

 p A'S indicate the position of the positive axis of the 

 mica and selenic films, and by this means results can 

 be recorded, &c. Either of the revolving cells can be 



FIG. 114. Swift's Diaphragms and Central Stops. 



thrown into the centre of the condenser, and there 

 stopped by means of a spring catch ; when so arranged 

 the mica film, &c., may be revolved in its place by 

 turning the cell D, asboth cells are geared together 

 with fine racked teeth. F is a polarising prism mounted 

 on an eccentric arm, rendered central when in use, or 

 thrown out, as seen when out of use. G is the rack dove- 

 tail slide for indicating focussing the condenser on 

 the object. The advantages of this condenser consist 

 in having the polarising prism, selenite films, dark 

 ground and oblique light stops, so that they may be 

 brought close under the optical combination. 



Collins's Webster's Universal Achromatic Condenser 

 (fig. 115) is a mechanical contrivance provided with 



