QUEKETT MICROSCOPICAL CLUB. 271 



chromatic plates in 1882 that light filters received serious attention 

 as a method of controlling contrast in photomicrography. Sanger 

 Shepherd & Co. put the first set of filters on the market in about 

 1900, and in 1904 described a set covering the whole range of the 

 visible spectrum. The Wratten M filters were issued in 1907, 

 a series by the Lifa Co. of Augsburg in 1917, a set for photo- 

 micrography by Ilford, Ltd., in 1919, and the Wratten visual 

 M filters in 1920. 



By the use of suitable filters the photographic rendering into 

 monochrome of the colour contrasts of a preparation may be 

 completely controlled. Resolution of fine structure may be made 

 easier by blue filters, the definition of achromatic objectives 

 may be improved by green, and the intensity of illumination 

 modified by neutral tints. If a colour is to be rendered as black 

 as possible it must be viewed or photographed by light which it 

 completely absorbs, i.e. by light of the wave-lengths comprised 

 within the dark parts of the spectrum obtained by viewing the 

 colour with a spectroscope. A conventional spectrum of blue, 

 green and red was then shown on the screen, and the extinction 

 of the various colours demonstrated by the interposition of 

 suitable screens. The position of the absorption bands is indicated 

 either by reference to the Fraunhofer lines or by a statement of 

 the wave-length of the light involved. In order to avoid blocking 

 of detail it is advisable to use a screen whose transmission 

 band is not centrally placed as regards the absorption band of 

 the colour in the preparation. As contrast is increased by the 

 use of a filter of the colour complementary to that of the prepara- 

 tion, so it may be reduced, if necessary, by using a screen of a 

 similar colour. This latter condition frequently presents itself 

 in the case of insect preparations that are too dark, and a red or 

 yellow filter may be used in these cases to bring out the detail. 



As regards the properties that should be possessed by a set 

 of light filters for use in microscopy, they should be : (1) efficient, 

 i.e. passing as high a percentage as possible of the light required ; 

 (2) convenient ; (3) permanent, under reasonable conditions, 

 and (4) there should be as few as possible consistent with affording 

 complete control of contrast. Coloured glass filters do not fulfil 

 the first condition, nor do they give clean-cut transmission bands, 

 otherwise they would fulfil the conditions. Liquid filters are 

 suitable for laboratory use, but, on the whole, nothing is so 



