110 



APPENDAGES TO THE MICROSCOPE. 



Fig. 60. 



for cutting -off the central rays in various degrees, three marginal 



slots for limiting the passage of 

 the illuminating rays to particular 

 parts of the periphery, and a 

 supplementary aperture for the 

 reception of any particular form of 

 stop or slot that the observer may 

 wish to employ. The edges of each 

 plate are stamped with figures, 

 which show what aperture is in 

 use in the Diaphragm-plate, and 

 what stop or slot in the Stop- 

 plate. It may be added that the 

 outer lenses of this combination 

 are removable ; so that two or 

 even one may be used alone, form- 

 ing a Condenser that is very suit- 

 able for use with Objectives of 

 medium power. 



Ross's Achromatic Condenser. 



80. Webster Condenser.— Though the original idea of the 

 arrangement which is now coming into general use under this 

 designation, and which is at the same time comparatively inex- 

 pensive and applicable to a great variety of purposes, was 

 given by Mr. J.Webster ("Science Gossip," April 1, 1865), it 

 has received important modifications at the hands of the Op- 

 ticians by whom the instrument is manufactured ; and has 

 perhaps, not even yet undergone its full development. In its 

 present form the arrangement of the lenses strongly resembles that 

 used in the Kellner Eye-piece (§ 22) ; 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 Achro- 

 matic combination, which consists of a double-convex lens of crown 

 with a plano-convex lens of flint, the plane side of the latter 

 being 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 



