New Way of producing Colour-Contrast . By J. Rheinberg. 383 
reach the eye. When we use a cone of illumination equal in aperture 
to the aperture of the objective, each part of the whole area of the 
objective back may be considered as receiving approximately an equal 
quantity of direct light and equal quantity of diffracted and refracted 
beams ; consequently, most light reaches the eye from the marginal 
zone of the objective, the light from the other zones being flooded out 
to a smaller or greater extent. With the photographic plate the same 
thing of course occurs. The practical result seems to be that we 
focus for the outermost zone of the objective, which receives direct 
light from the condenser; and this probably, to a certain extent, 
explains why few objectives of high power will admit of a full cone of 
light being used. 
But this is somewhat of a digression from the main subject. 
Let me now refer to the method of optical colouring available with 
low powers. 
This is an extension, as I have before mentioned, of ordinary 
dark-ground illumination. 
We place in the diaphragm carrier of the condenser coloured discs 
similar to those for use above the objective, only of larger size, and 
in this case the central portion of the disc, which we will suppose to 
be blue, must be sufficiently large that the cone of light it transmits 
completely fills the aperture of the objective (plate IX. fig. 3). The 
red periphery should be of the full aperture of the condenser. Using 
a cone of light as large as the condenser will permit we see the object 
red on a blue background. In the previous method we utilised the 
central pencils of light, which impinging on the object at a relatively 
small angle from the normal, are bent outwards into other zones of 
the objective ; in this method we chiefly use the oblique illuminating 
pencils from the condenser which the object bends inwards so that 
they can be taken up by the objective (plate IX. fig. 4). 
So far, the action of the two is somewhat analogous, but the 
conditions determining the result in the present method are much 
simpler than in the former one, for they resolve themselves into a 
mere question of the relative quantity of light of the different colours 
which impinge on the object. 
If the diameter of the central blue zone is one-third the diameter 
of the whole disc, the difference in area between it and the red portion 
is so great that the relatively small percentage of the former scarcely 
affects the general colour of the object visibly. 
With this method we can also get many further results by using 
discs with the colours differently arranged. Supposing we use a disc 
with the central red and the periphery divided into four quadrants, 
the two opposite ones being blue and the other two yellow (fig. 1 g ), 
and look at an object having ridges or striations at right angles to 
one another, then, taking care to bring the disc to the correct position, 
the one set appears blue, the other yellow, the whole on a red back- 
ground. 
