176 THE MICROSCOPE. 



out the lines on Navicula Angulata^ Ceratoneis Fasciola, 

 Amician Test, Grammatophora Subtilissima, and even on 

 the Navicula Acus, with the greatest ease. In viewing 

 these delicate objects, the lines must be at right angles 

 with the direction of the light. If the angle c b a be 

 greater than half the angle of aperture of the object lenses, 

 then you have the object beautifully illuminated on a 

 black ground ; and when the light is properly managed, 

 the result is very superior to that obtained by the parabolic 

 reflector — the objects on the black-ground appear as though 

 they were in full sunhght, bringing out the fine markings 

 on delicate tissues most satisfactorily.' 



" My next trial was made with a small rectangular 

 achromatic prism with a focus of two inches and a half — 

 not of four or five inches, as in Mr. Sollitt's case — fur- 

 nished by Messrs. Powell and Lealand ; and I then found 

 it easy to resolve the markings of all test objects which 

 were disposed at right angles with the source of illumina- 

 tion. With a view of resolving the crossed-lines of F. 

 Formosum, N. Angulata, and the like, I employed the 

 usual microscope mirror, so disposed by means of a double- 

 jointed arm that the light from the lamp might be reflected 

 as nearly as possible at right angles with the second set of 

 lines, but met with but moderate success, since the double 

 reflection, due to the primary reflection from the silvered 

 surface of the mirror, and the secondary reflection from 

 the glass of -the mirror itself, caused a haze that militated 

 materially against true definition of the markings which 

 characterise such delicate tests as N. Rhomhoides, P. Ma- 

 crum^ &c. 



" The next step was to substitute for the mirror a second 

 lamp and a second prism, also mounted on a separate stand, 

 with rack- work adjustment, and provided with a universal- 

 joint arm. The success of this plan was undoubted ; I 

 was now enabled to resolve the crossed lines of N. Rhom- 

 hoides (the Amician Test of the London, although, per- 

 haps, not of the American microscopists) with a sharpness 

 which was hitherto unattainable ; and even the diflicult 

 object P. Macrum came out in well-defined squares. The 

 advantage of the universal-joint fitted to the second pi'ism 

 consists in the fact that the diagonal lines of such tests as 



