New Way of producing Colour-Contrast, By J. Rlieinberg. 387 
ing the object, or even of particular structure in the object. At all 
events, interesting results from a theoretical point of view may he 
looked for by the application of the colour-discs to photomicrography, 
and I am convinced that an extended series of experiments in this 
direction will well repay the time spent. 
It may not he amis3 to describe briefly how the colour-discs may 
be prepared. 
For the condenser colour discs or for the stand-up screens, glass 
painted in the required manner with coloured gelatin can be employed. 
If desirable, another thin glass can be cemented with Canada balsam 
over the gelatin film, to prevent the liability of scratches. For the 
discs to be used with the objective, however, gelatin films will not 
do, as they cannot be got sufficiently homogeneous. For these it is 
preferable to coat the glasses with coloured collodion. This may be 
made by dissolving a small portion of colouring substance such as 
fuchsin, methylen-blue or malachite-green in alcohol to which, after 
filtration, an equal quantity of ether is poured. To this is added 
sufficient pure collodion to give a thin film on glass (generally an 
equal quantity to the above mixture). If the collodion when applied 
to the glass peels off, more alcohol should be added ; if the film cracks 
when drying, more ether must be added. Microscopic cover-glasses 
of the required size are now coated with the coloured mixtures, and 
supposing, for example, that we want to make a disc with 2 mm. 
diameter blue centre and red periphery, we should take a red disc and 
scratch off the collodion film with a needle point from the central 
zone of 2 mm. diameter, and from a blue disc we should remove the 
film from all except that central part. Then the two glasses can be 
cemented together with Canada balsam with their filmed surfaces in 
contact. 
For experimental purposes, where great durability is not required, 
we can coat a single piece of thin glass one colour one side, the other 
colour on the opposite side, and then scratch off the films where not 
wanted. When absolute homogeneity of the film is required, it is 
better to coat large sheets of thin glass with the collodion, nnd 
subsequently cut the circular discs from these. 
Another way of making the discs is to insert a circular piece of 
glass of one colour into a corresponding drilled hole in another, and to 
grind the two together to ensure plane parallel surfaces, otherwise we 
obtain a lateral displacement of images from the two parts as in the 
dou.ble-image colour-discs. A better way than both of these, un- 
doubtedly the best way in fact, would be to stain glass directly in 
various colours, if this can be done. 
Besides homogeneity of the discs, one other requirement will be 
necessary to obtain the finest results with high-power work, viz. to 
make some provision for the alteration in the correction of the lens by 
the interposition of the glass discs above (or between) the lenses of 
the objective. If it were not too expensive and did not necessitate 
