ILLUMINATING APPARATUS 43 



When discussing the aperture of an object glass, the resolution 

 was stated to be dependent on its aperture, but it is also depen- 

 dent on the colour of the light. White light, when split into its 

 component parts by, for instance, a rainbow or a prism, con- 

 sists of certain pure spectrum colours — red, yellow, orange, green, 

 blue, and violet. The resolution obtained with white light is that 

 due to the orange-coloured portion of its component parts, because 

 this coloured light is more powerful than any of the others that 

 go to make it up. If a green light be used, the resolution of a 

 microscope can be increased about 15 per cent., and if a purple 

 be used, about 25 per cent. 



A purple light, or even a very dark blue, is unpleasant to the 

 eyes of most observers, but a bluish-green light is very restful, 

 and is the best colour to use as regards 

 microscope resolution. 



Apochromatic object glasses are so 

 perfectly corrected for colour that the 

 brilliancy of their images, quite apart 

 from resolution, will not be improved 

 by the use of colour screens ; but 

 achromatic object glasses are always 

 slightly, and under some conditions 

 considerably, improved in their per- 

 formance by the use of a screen 

 which transmits a pure colour. 



The green glass supplied with 

 the substage condensers trans- 

 mits a moderately pure colour, 

 but is not so good as the special 

 Wratten &; Wainwright gelatine 

 filters. 



A glass trough about 3/4 inch 

 thick, filled with a nearly saturated solution of acetate of colour 

 copper, makes a fairly pure blue-green screen. It is rather more ^^°^^^' 

 transparent than a gelatine filter. 



Various fluid screens have been used, but they are so much less 

 convenient than the Wratten gelatine screens that they are not 

 so frequently employed. 



The best form of illuminant for the microscope depends upon sources of 

 many circumstances. The author is of opinion that daylight is illumination. 

 the worst form for accurate observation, but that when it is used 

 a screen or card with a hole in it about 2J inches diameter should 

 be placed in front of the mirror of the microscope about 8 or 

 10 inches away. This ensures a moderately parallel beam of light 

 falling upon the mirror. A paraffin lamp with a flat flame is 

 probably the most convenient light for general purposes, 

 but it is not powerful enough for the use of colour screens or for 

 high-power dark-ground illumination. The electric light of 



Fig. 35.— No. 3366, Monochro- 

 matic Light Trough. 



