CONDENSERS. 81 



It is due to Mr. Lister to mention that in his paper on 

 the "Achromatic Object- Glass," published in the 120th 

 vol. of the Transactions of ike Royal Society, he makes 

 mention " of some objects being better seen when the 

 central rays are obscured." This observation has been 

 carried out in many ways. Mr. Reade's "back-ground 

 illuminator" is one in which the light is thrown under the 

 object in such a direction as to avoid or pass by the aper- 

 ture of the object-glass, and give a black field. The 

 structure under view, if large, must have sufficient trans- 

 parency to allow the light to enter into its substance, and 

 to be diffused or radiated therefrom in all directions. 

 This illuminator is very suitable for objects requiring only 

 a low power to view them. 



Mr. John Furze directed the attention of microscopists 

 to a beautiful arrangement for the "illumination of objects 

 by polarised light on a dark field, in such a manner as to 

 give the object a stereoscopic effect by a due contrast of 

 light and shade." To obtain this result, he uses a plano- 

 convex lens, three-fourths of an inch in diameter. This, 

 when fitted, is of so small a size, that it can be adapted to 

 any instrument. Such an illuminating lens should be 

 arranged with a system of both central and external stops, 

 each revolving on a separate axis; and an adjustable cap 

 to slide over the top of the lens, containing a crystal of 

 Herapathite mounted between thin glass; a plate of selenite, 

 mounted in the same 'way, should be used on the stage 

 above it. Objects of too great density for transmitted 

 light will appear under this mode of illumination as if in 

 relief; and the definition of the various parts will be so 

 accurately displayed as to constitute a most perfect method 

 of viewing them. 



Condensing lenses, fig. 45, are used either for opaque 

 objects, or to condense the light upon the mirror attached 

 to the microscope. A bull's-eye, or plano-convex lens, of 

 three inches focal length, is best suited for the purpose. 

 In fig. 46 the bull's-eye lens c slides up and down a brass 

 rod, screwed into a steady foot; or it may be fixed into 

 the stage of the microscope, through which the light is 

 finally concentrated upon the object from the table gas- 

 lamp d. Mr. Brooke's method of viewing opaque objects 



a 



