42 



THE MICROSCOPE 



Colour 



screens. 



than the Beck illuminator, but most prefer the thin glass form 

 for high powers, and the Sorby reflector for low-power work. 

 As a convenient and universal illuminator for all powers where 

 the highest resolution is not required, the prism illuminator, 

 especially with an electric light bulb attached to it, is popular 

 for metallurgical work. Fig. Ill (p. 120) shows this illuminator 

 provided with a small focussing lens, a receptacle for colour 

 filters, and a 16-candle-power electric light to suit either the 

 100- or 200-volt current in a metal casing. 



Colour screens are of use for several important purposes. 

 They are either coloured glasses or coloured gelatine mounted 

 between two glasses. They give greater contrast where objects 

 being examined are stained or are naturally coloured, and 



give truer rendering of 

 natural colours where 

 artificial light is em- 

 ployed. 



Every substage con- 

 denser should be sup- 

 plied with a green glass, 

 but a set of difierent 

 coloured screens is 

 very useful for increas- 

 ing colour contrasts, 

 both for visual and 

 photomicrographic work. 

 If a specimen of 

 bacteria is stained 

 faintly with red, the 

 use of a green screen 

 will make them appear almost black and much more distinct. 

 If a specimen is stained — 



Blue, a red filter should be used. 



Green, a red „ 



Red, a green „ 



Yellow, a blue „ 



Brown, a blue „ 



Purple, a green „ 



Violet, a yellow „ 



If the screen is too dark, and the light and shade contrast 

 too great in consequence, fine detail in the structure may be 

 somewhat clogged or obscured, but faintly stained or coloured 

 specimens are rendered much more visible by the use of the 

 correct colour filter. 



Another marked advantage in the use of colour filters of green 

 or blue is obtained by the greater power they give to an object 

 glass of resolving fine structure. 



