182 THE MICROSCOPE. 



light may be tlirown upon the object, and in any particular 

 direction. In this way points of structure have been 

 brought out which were lost under the full illumination of 

 the Lieberkuhn. Mr. Beck sought to effect an improve- 

 ment upon the proposal of Mr. Bridgenian, and, by 

 employing the segment of a Lieberkuhn, produced what he 

 terms a "Parabolic reflector." This is made to slide in the 

 ordinary way over the object-glass, and admits of being 

 turned in any direction towards the source of light. The 

 parabolic reflector can also be employed with high powers. 

 Mr. Sorby discovered, while experimenting with a reflector 

 of the kind, the value of studying the peculiarities of 

 objects under every kind of illumination ; for, on viewing 

 specimens of iron and steel with this reflector, he found 

 that, owing to the obliquity of illumination, the more 

 brilliantly polished parts reflected the light beyond the 

 aperture of the objective, and he could not therefore 

 distinguish them from those parts which merely absorbed 

 the light. To throw the illumination more perpendicularly 

 upon the specimen, he was obliged to place a small flat 

 mirror immediately in front of the objective, and cover half 

 its aperture, and at the same time stop-off, by means of a 

 semi-cylindrical tube, the light from the parabolic reflector. 

 By such an arrangement, Mr. Sorby found it produce the 

 reverse appearances of the former mode of illumination, 

 and it proved to be a valuable aid in ascertaining the true 

 condition of the object. Mr. Baker has produced a cheaper 

 and simpler form of this parabolic reflector, which answers 

 its purpose remarkably well, giving increased facilities for 

 the illuniination of opaque objects with the microscope 

 set in that direction most convenient for working. The 

 brilliancy of the illumination is very much increased by 

 placing a condensing-lens before the light, or by employing 

 the Bockett lamp. It is, however, only just to Mr. Ross 

 that we should add, that this " parabolic reflector " is 

 but a slight modification of his Side-reflector, contrived 

 several years ago, for the purpose of superseding the 

 Lieberkuhn. 



The Bull's-eye Condenser. — All opaque objects require to 

 be illuminated by rays which, being thrown upon their sur- 

 face, shall be reflected back into the microscope. The sarc^ 



