New Way of producing Colour- Contrast. By J. Rheinberg. 381 
• acknowledge my indebtedness to the learned professors at the firm of 
Carl Zeiss, of Jena, who have during the past year, with great 
courtesy and generosity, lent me their valuable assistance and advice 
in the furtherance of this system of illumination, and to whom certain 
modifications of the method for low-power work, and especially the 
following ingenious ocular demonstration of princqfies involved in 
the Abbe theory, are due. 
They have made some colour-discs of green glass, with a small 
central red glass of 2 mm. or 3 mm. diameter inserted ; the one part 
of the disc, however, instead of having plane parallel faces, is 
slightly prismatic in form (plate IX. fig. 1 h), and effects a displace- 
ment in the image as compared with the other part. By using such 
a glass in the back focal plane of the objective, we obtain two images 
of any object side by side, the one formed by the red centre, the other 
by the green rim. 
The nature of these two images I have already explained, and 
you will readily understand, therefore, that by the present arrange- 
ment we see through the Microscope at one and the same time a red 
image on a red background of a suitable diatom showing plenty of 
coarse structure and very little fine structure, next to a green image 
of the diatom, with fine detail and somewhat unclear in respect of its 
coarser structure. This affords a singularly striking ocular proof of 
the fact first pointed out by Prof. Abbe, that a single dioptric (or 
single diffraction pencil) will not show any structures ; whilst, when 
at least two of the emergent pencils from any single elementary 
pencil impinging on fine structure reach the eye, such structure 
becomes visible. Discs as described, which I will call double image 
colour-discs, are also of great interest in showing the relative part 
which the different zones of the objective play in the formation of the 
image, looked at from another point of view, viz. according to the 
intensity of light proceeding to the eye from the different zones under 
varying conditions. I will show you a set of four photomicrographs, 
all taken with a 1-in. objective under precisely the same conditions, 
with exception that the opening of the iris diaphragm was varied. 
In taking the first one (plate X. fig. 1) the iris was closed to 
2J mm. diameter, under which condition only the red central part of 
the colour-disc could receive direct light. The background was red, 
therefore, and appears dark coloured on the photograph in con- 
sequence. In strong contrast to the background we see the white 
images A, formed by the green peripheral part of the disc, showing 
plenty of detail, though some of them are wanting in clearness of 
outline, and the dark images B, which show a strong outline, but 
are wanting in all but the coarsest structure. 
Next we have the same (plate X. fig. 2) taken with a 6 mm. 
iris opening, under which condition not only the red central part of 
the disc, but also a narrow ring of the green part, would receive 
direct light. The colour of the background will now have a certain 
