bull's-eye condensek. 



143 



ta^eous to use the achromatic condenser in combination with it ; 

 this combination, which cannot be made in ordinary microscopes, 

 is provided for in that of Messrs. Smith and Beck, by the " cyhn- 

 drical fitting" so often referred to, which can receive the polar- 

 izing prism at its lower end, and the achromatic condenser at its 

 upper, whilst the selenite plate or plates may be interposed be- 

 tween them.' 



64. Illuminators for Opaque Objects. — All objects through which 

 sufficient light cannot be transmitted to enable them to be viewed 

 in the modes already described, require to be illuminated by 

 rays, which, being thrown upon the surface under examination, 

 shall be reflected from it into the microscope ; and this mode of 

 viewing them may often be 

 advantageously adopted in Pm 44 



regard to semi-transparent or 

 even transparent objects, for 

 the sake of the diverse aspects 

 it affords. Among the various 

 methods devised for this pur- 

 pose, the one most generally 

 adopted consists in the use of 

 a condensing lens, either at- 

 tached to the microscope, or 

 mounted upon a separate 

 stand, by which the rays pro- 

 ceeding from a lamp or from 

 a bright sky are made to con- 

 verge upon the object. For 

 the efficient illumination of 

 large opaque objects, such as 

 injected preparations, it is 

 desirable to employ a " bull's- 

 eye" condenser ' (which is a 

 plano-convex lens of short 

 focus, two or three inches in 

 diameter), mounted upon a 

 separate stand, in such a man- 

 ner as to allow of being placed 

 in a great variety of positions. 

 The mounting shown in Fig. 

 44 is perhaps one of the best Buii's-Eje oondeuser. 



that can be adopted : the 



frame which carries the lens is borne at the bottom upon a swivel- 

 joint, which allows it to be turned in any azimuth; whilst it-may 

 be inclined at any angle to the horizon, by the revolution of 



^ For an account of the nature and properties of Polarized Light, which would he 

 out of place in the present treatise, see the chapters on that subject in Dr. Golding Bird's 

 " Manual of Natural Philosophy," Dr. Pereira's " Lectures on Polarized Light," New Ed., 

 edited by Prof Baden Powell, or any modern treatise on Optics. 



