64 THE MICROSCOPE. 



fracted according to their inclination, and proportionate to the various 

 densities of the medium, the most refractive structure transmitting the 

 greatest quantity of light, and being in consequence the most illumi- 

 nated; and this reason is somewhat confirmed by the circumstance 

 of lateral illumination showing the structure of some objects which, 

 from slight variation in density, were invisible, except by the use of 

 polarised light. Mr. Shadbolt has since modified this reflector, which 

 he denominates " a sphero-annular condenser :" it has superior reflect- 

 ing arrangments, with less liability of derangement, and is constructed 

 of a solid cylinder of glass terminating above in a solid cone, the sur- 

 face of which has the form of a parabola, and replaces the silver reflect- 

 ing surface. 



It is due to Mr. Lister to mention that in his paper on the " Achro- 

 matic Object-Glass," published in the 120th vol. of the Transactions of 

 the 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. The Rev. 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 aperture 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 suit- 

 able for objects requiring a low power to view them. 



Condensing lenses are used either for opaque objects, or to condense 

 the light upon the mirror attached to the microscope. Two lenses, as 

 represented in fig. 42, are sometimes used. A bull's-eye, or plano-convex 

 lens, of three inches focal length, is best suited for the larger ; and the 

 mode of employing the two condensers upon an object placed on the 

 stage of the microscope at a is here shown. The bull's-eye lens c slides 

 up and down a brass rod, screwed into a steady foot ; 6, the smaller lens, 

 working on a joint, 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 under 

 the highest powers of the microscope (the ^ and ^ inch object-glass) is 

 effected by two reflections. The rays from a lamp rendered parallel by a 

 condensing lens are received on an elliptic reflector, the end of which is 

 cut off a little beyond the focus ; the rays of light converging from this 

 surface are reflected down on the object by a plane mirror attached to 

 the object-glass, and on a level with the outer surface. By these means 

 the structure of the scale of the Podura, and the different characters of 

 its inner and outer surfaces, are rendered distinctly visible. Silver 



