154 SELECTION AND USE 



lenses strongly resembles that used in the Kellner Eye-piece; 

 the field-glass of the latter serving as a condenser to receive 

 the cone of rays reflected upwards from the mirror, and to 



make it converge upon 

 a smaller achromatic 

 combination, which 

 consists of a double- 

 convex lens of crown, 

 with a plano-convex 

 Fig. 45. WEBSTER CONDENSER. lens of flint, the plane 



side of the latter be- 

 ing next the object. These lenses are of large size and deep 

 curvature; so that when their central part is stopped out, the 

 rays transmitted from their peripheral portion meet at a wide 

 angle of convergence, and have the effect of those transmitted 

 through the peripheral portion of the ordinary achromatic 

 condenser. When, on the other hand, this combination is used 

 with a diaphragm that allows only the central rays to pass, 

 these rays meet at a small angle; and the illumination thus given 

 is very suitable for objects viewed with low powers. Again, by 

 stopping out the central portion of the combination, and re- 

 moving the condenser to a short distance beneath the object, 

 the effect of a black ground illumination can be very satisfac- 

 torily obtained with objectives of moderate angular aperture. 

 Further, by stopping out not only the central, but also a great 

 part of the peripheral rays, so as only to allow the light to enter 

 from a small portion or portions of the margin, oblique illumin- 

 ation can be most effectively obtained." 



The Wenliam Reflex Illuminator. This is gener- 

 ally conceded to be the most perfect device for illuminating 

 balsam-mounted objects when viewed by objectives of very 

 high angles. It is shown in section in Figure 46, where a is a 

 cylinder of glass half an inch long, and four-tenths in diameter, 

 the lower convex surface of which is polished to a radius of 

 four-tenths. The top is flat and polished. Starting from the 

 bottom edge, the cylinder is worked off to a polished face at an 

 angle of 64. Close beneath the cylinder is set a plano-convex 

 lens of li inch focus. Parallel rays,/,/,/, sent through the 

 lens, after leaving the lower convex surface of the cylinder, 



