116 SELECTION AND TTSE 



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. 24. 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 Spot Lens. This is a plano-convex lens of very high 

 curvature, (it is generally hemispherical,) so mounted that its 

 distance from the object may be properly adjusted, and in this 

 way the rays which pass through it may be brought to a focus 

 on the object. The central rays are stopped out by means of a 

 black spot, (hence the name,) so that the object is illuminated 

 wholly by rays which are of too great obliquity to enter the 

 object-glass, except when their direction is changed by the 

 object. The latter, therefore, appears brilliantly illuminated on 

 a dark ground, and in many cases features which could not 

 otherwise be seen are shown very distinctly. 



