116 



APPENDAGES TO THE MICROSCOPE. 



be employed ; and when, instead of the parallel rays of Daylight, 

 we are obliged to use the diverging rays of a Lamp, these should 

 be rendered as parallel as possible, previously to their reflexion 



from the mirror, by 

 the interposition of 

 the ' bull's-eye ' Con- 

 denser (§ 90) so ad- 

 justed as to produce 

 this effect. There 

 are many cases, how- 

 ever, in which the 

 stronger light of the 

 concave Mirror is 

 preferable. When 

 it is desired that the 

 light should fall on 

 the object from one 

 side only, the cir- 

 cular opening at the 

 bottom of the wide 

 tube (Fig. 63) that 

 carries the Pai-a- 

 boloid may be fitted 

 with a diaphragm 

 adapted to cover all 

 but a certain portion 

 of it ; and by giving 

 rotation to this dia- 

 phragm, rays of 

 great obliquity may be made to fall upon the object from every 

 azimuth in succession. 



86. For the exhibition of those classes of objects which are 

 suitable for Black-Ground illumination, and which are better 

 seen by light sent into them from every azimuth than they are 

 by a pencil, however bright, incident in one direction only, no 

 more simple, convenient, and efficient means could probably be 

 found than that which is afforded by the Spot-Lens or the Webster 

 Condenser for Objectives of low power, and by the Parabolic 

 Illuminator for powers as high as l-4th or l-5th of an inch focus ; 

 the use of the latter with powers still higher being rendered dis- 

 advantageous by the great reduction in the amount of light, 

 occasioned by the necessity for cutting-off all the rays reflected 

 from the Paraboloid which fall upon the object within the limits 

 of their angle of aperture. With Objectives of extremely wide 

 angles of aperture, a black -ground illumination may be obtained 

 by Mr. Reade's Double Hemispherical Condenser (§ 83). — One of 

 the great advantages of this kind of illumination consists in this ; 

 that, as the light radiates from each part of the object as its 



