78 PRACTICAL MICROSCOPY. 



when this is pushed upwards it cuts off the less divergent 

 rays which would otherwise proceed from the apparatus. 

 When lamplight is used, the bull's-eye condenser should 

 be interposed between the lamp and the plane mirror, in 

 order to parallelise the rays which fall upon the latter. A 

 large mirror is requisite for the proper manipulation- of the 

 paraboloid, and this is not usually obtained in third-class 

 instruments. 



When objectives of wide angle are used, a somewhat 

 different arrangement is necessary to secure dark-ground 

 illumination. In the ' Monthly Microscopical Journal ' for 



July 1869, Mr. Wenham 

 reproduces the illumina- 

 tor which he first described 

 in 1856 as "A method 

 of illuminating opaque 

 objects under the highest 

 powers of the Micro- 

 scope." Fig. 60 will illus- 

 trate the subject of these 

 remarks. Cemented to a 



glass slide with Canada-balsam is a nearly hemispherical lens 

 " with a segment removed so as to leave the thickness equal 

 to about one-third the diameter of the sphere. The circle 

 formed by the removal of the segment is blackened in order 

 to exclude all rays below the incident angle of total reflection. 

 This lens is intended to be used in conjunction with the 

 paraboloid as shown in Fig. 60. The rays pass through the 

 lens in a radial direction without refraction, and proceed 

 till they reach the upper surface of the thin glass cover, 

 where they are totally reflected upon the object." 



An immersion paraboloid has since been devised by 

 Dr. Edmunds, and made by Messrs. Powell and Lealand ; 

 it is somewhat similar to the above in principle. 



