WITH THE MICROSCOPE. 23 



33. The Parabolic Reflector of Mr. Wenham, Mr. Shadbolt's 

 annular condenser, and the parabolic illuminator of Messrs. Smith and 

 Beck are beautiful instruments for effecting the same purpose in a 

 more efficient manner, pi. XII, fig. 55. Another excellent plan has 

 lately been devised by Mr. Wenham, the simplicity of which recom- 

 mends it strongly to our attention. A small triangular prism is 

 placed beneath the object, so that one of its plane surfaces is in 

 contact with the under surface of the slide carrying the object. The 

 light is refracted so highly that none passes directly through the 

 object, but, being thrown at the proper angle upon the under surface 

 of the thin glass which covers it, is entirely reflected from thence 

 upon the object itself, which is thus highly illuminated. 



Of examining the Internal Structure of Objects by Transmitted Light. 



34. Transmitted Light. In discussing the mode of illuminating 

 objects by transmitted light, I must briefly draw attention to two 

 or three beautiful instruments for condensing the light upon the 

 object. The microscope in pi. X, fig. 42, is arranged in the ordi- 

 nary position for examining transparent objects. The light may 

 be received upon the plane or concave mirror, according as a 

 moderate or brilliant light is required ; but, as a general rule, the 

 intensity of light should not be greater than necessary to make out 

 distinctly the structure of the object. Direct sunlight is not to be 

 employed, and a very strong light of any kind is hurtful to the eyes. 

 The best light during the day is to be obtained from a white cloud 

 upon which the sun is shining. 



35. Monochromatic Illumination. Professor Amici seems to have 

 been the first to have tried experiments with monochromatic light in 

 the examination of objects in the microscope. He employed the 

 rays of the solar spectrum, but I am not aware that any great advan- 

 tages have been obtained or new facts discovered by this process. 

 Yet it seems probable, now that we are enabled to examine objects so 

 much more minutely than heretofore, that something may be gained 

 by enquiries in this direction. Any ray from an ordinary prism may 

 be caused to pass through the object or condensed upon it with the 

 aid of the condenser. Count Castracane has since used mono- 

 chromatic light for microscopical observation and for taking micro- 

 scopical photographs (Microscopical Journal, October, 1865, p. 250), 

 and a similar plan has also been used in America by Mr. L. W. Ruther- 

 furd for the same purpose. Count Castracane used one of Dubosq's 

 Heliostats, a rather expensive instrument, by which the whole field 

 could be illuminated by any single ray desired, and by the move- 



