ZOOLOGY AND BOTA'NY, MICROSCOPY, ETC. 



451 



catch had to be added in order to prevent the sinking of the tube. For 

 this reason a separate stand was soon employed, which had, however, the 

 effect of introducing objectionable complications, viz. a difficulty in 

 adjusting the liuht-proof connection between Microscope and camera, 

 and in avoiding shakings which affected unequally the two parts of the 

 apparatus. The author cousiders that to the latter cause is to be 

 ascribed a very large proportion of the failures in photomicrography, 

 and from this his apparatus is free. 



As the source of illumination, the author employs almost exclusively 

 the Welsbach incandescent gaslight. The lamp is placed as near as 

 possible (20-25 cm. in front of the mirror), and always without the 

 interposition of lenses, so that the object is illuminated by transmitted 

 light. The objection made by ISTeuhauss to the use of transmitted light, 

 that shadows and coloured margins were produced, was not borne out by 

 the author's observations. 



Another source of error, however, viz. the difference between the 

 visual and chemical foci, must in all circumstances be taken into account. 

 This difference varies considerably for lenses of different construction ; 



Fig 68. 





in immersion lenses it is so small as to be negligible, but the lower dry 

 objectives show it without exception and require corresponding correction 

 by the use of monochromatic-blue light. 



The focal adjustment of the image in the camera is made on a glass 



