60 LIGHTING AND FOCUSING [ CH. II 



This gives fairly good results with all powers. The same may also 

 be accomplished, as shown by Gordon, 1906, by using a stop in the 

 eye-point or Ramsden circle ( 67). 



For a discussion of dark-ground illumination and ultrami- 

 croscopy see : A. E. Wright, Principles of Microscopy, Ch. XIV; 

 Siedentopf, Jour. Roy. Micr. Soc. 1903, p. 573, 1907, p. 733 ; 

 Gordon, 1906, p. 167 ; Beck, 1908, p. 238 ; Reichert, 1908, p. 374 ; 

 Leitz, 1905, p. 502 and Catalog No. 42, and special catalog. Top- 

 pier, Poggendorff's Annalen, 1867, p. 33 ; Beck's Cantor Lectures, 

 1907 ; Zeiss special catalog on Ultramicroscopy and dark-ground 

 illumination, 1907, gives the apparatus needed, the methods and 

 application, also bibliography ; Cotton et Mouton, Les Ultrami- 

 croscopes, Paris, 1906. 



ARTIFICIAL ILLUMINATION 



104. For evening work and for regions where daylight is 

 not sufficiently brilliant, artificial illumination must be employed. 

 Furthermore, for the the most critical investigation of bodies with 

 fine markings like diatoms, artificial light has been found superior 

 to daylight. 



A petroleum (kerosene) lamp with flat wick gives a satisfactory 

 light. It is recommended that instead of the ordinary glass chim- 

 ney one made of metal with a slit- opening covered with an oblong 

 cover-glass is more satisfactory, as the source of light is more 

 restricted. Very excellent results may be obtained, however, with 

 the ordinary bed-room lamp furnished with the usual glass chimney. 



The acetylene light promises to be excellent for microscopic 

 observation and for photo-micrography. (See under photo- 

 micrography.) See also ic>3a. 



Whenever possible the edge of the flame is turned toward the 

 microscope, the advantage of this arrangement is the great bril- 

 liancy, due to the greater thickness of the flame in this direction. 



105. Mutual Arrangement of Lamp, Bull's Eye and 

 Microscope. To fulfil the conditions given above, namely, that 

 the object be illuminated by the image of the source of illumination 

 the lamp must be in such a position that the condenser projects a 

 sharp image of the flame upon the object (Fig. 60), and only by 

 trial can this position be determined. In some cases it is found ad- 



