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Transactions of the Society. 
background of any other colour, and by thus securing a greater 
contrast than usual, to obtain sharper and more perfect definition. 
That any increase of contrast, produced without materially altering 
other conditions, does sharpen the visual perception of an object when 
viewed through the Microscope goes almost without saying, since it 
is an admitted fact of the nature of an axiom that all vision is depen- 
dent upon the perception of contrast. 
I hope to be able to show you that I have succeeded in obtaining 
this desired result with numerous classes of objects, and that by the 
same methods by which it is possible to effect a colour-contrast 
between the background and the object, it is also, under certain 
circumstances, possible to effect a colour-contrast between various 
definite parts of the objects themselves. 
The way in which these results can be obtained is to place trans- 
parent discs or screens (plate IX. fig. 1), the central and peripheral 
parts of which are differently coloured, or which are coloured in certain 
parts only, somewhere in the path of the rays of light proceeding from 
the source of illumination. 
The discs to be employed are for the most part made up of several 
pieces of glass, or, if preferred, of one piece of glass only, coated with 
collodion or gelatin of the required colours. The most generally useful 
discs are those in which the central portion is of one colour, e.g. red, 
while the peripheral portion is of another well contrasting colour such 
as green or blue (plate IX. fig. 1 c), and those in which the central or 
peripheral part only is coloured (plate IX. fig. 1 a b). The breadth of 
the central portion should be from about one-third to one-fifth of 
the effective diameter of the disc. 
According to the circumstances of the case such discs of suitable 
size may be placed somewhere between the back lens and the focal 
plane of the objective, or in the diaphragm-holder of the condenser, or 
interposed between the illuminant and f the mirror. Sometimes even 
it is useful to place them between the lenses of the objective. 
It will have now become evident that this colour-disc method is 
solely concerned with optical effects, so that before considering it in 
greater detail it will be well briefly to define its relation to several 
optical methods of changing the colour of object or background which 
are in general use. 
In the first place may be mentioned monochromatic illumination, 
produced either by spectroscopically decomposing white light, or by 
placing an absorption screen in the path of the illuminating cone. 
The advantage gained hereby for visual work lies chiefly in the 
elimination or reduction of chromatic aberration, in addition to which 
a more sharply defined image of the finer structure is obtained, since 
light of approximately uniform wave-length is substituted in the place 
of light possessing wave-lengths of various magnitudes, which latter, 
in the case of fine structures giving rise to extended diffraction-fans, 
produce separate final images which vary in their nature and position. 
