OBLIQUE ILLUMINATION. 171 



fittings, and a reflection from the back of the object- 

 glass-lenses, that an unfavourable opinion was formed of 

 its practicability. It has since been demonstrated that 

 the light was too intense ; and the most useful, the central 

 portion of rays, were wanting. A simple disc of thin 

 glass and its partial reflection meets these objections. If 

 such a disc be used with a little care, it is found to be 

 quite as accurate as the other plan, and the natural surface 

 of the glass is a better reflector than any artificial one. It 

 has, however, the disadvantage of extreme fragility. By 

 making the object-glass its own condenser, and examining 

 diatoms as opaque objects under high powers, we may 

 now hope to solve the much vexed question as to the true 

 nature of their markings. Mr. Browning has employed 

 the apparatus in a form much more nearly resembling that 

 of the original inventor, only substituting a small glass 

 reflecting-prism for the metallic reflector. Some advan- 

 tages are gained by the adoption of this latter arrangement. 

 Such modes of illumination bid fair to correct many errors 

 of interpretation resulting from an exclusive use of trans 

 parent illumination. 



Oblique Illumination is of the greatest importance in 

 the demonstration of structure, especially that of test- 

 objects ; and much greater obliquity is often required 

 than can be obtained by throwing the mirror out of the 

 axis of the microscope. Various methods are described ih 

 the "Transactions" of the Microscopical Society of London, 

 for almost every microscopist has devoted some attention 

 to the subject in his attempts to resolve the more diffi- 

 cult lined or dotted objects, as diatoms ; therefore, to 

 give a full description of all is quite uncalled for, if 

 not impossible, in our pages ; it is nevertheless most 

 advisable, in doubtful cases, to have recourse to every 

 method which shall present the object under a different 

 aspect. One of the earliest methods devised for obtaining 

 oblique light, was the eccentric prism of Nachet, which 

 occupied the place of the present achromatic condenser, 

 and, like it, received its light from the mirror ; by simply 

 turning it in its socket, oblique rays were thrown upon the 

 object from every side. This had, however, defects which 

 Mr. Soliitt proposed to remedy, by employing an achro- 



