70 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
struction of this instrument advantage has been taken of the fact that the 
eye is unable to analyze a series of impressions which follow in such rapid 
succession that each impression lasts less than about one-tenth of a second. 
The optical "mixer," as it is called, in this instrument consists of a rotating 
wheel, to the rim of which are attached, at equal intervals, lenses of uniform 
focal length, which, together with the low-power eye lens and a cylindrical 
lens, project an image of the colored apertures in the eye-point. A small 
electric motor drives the wheel at uniform speed, so that the three colored 
apertures pass the eye repeatedly in such quick succession that resolution 
is impossible and the color resulting from the mixture of the three colors 
appears uniform. By means of properly adjusted shutters, the requisite 
amount of light of each standard color can be made to enter the system, 
thus obtaining any desired color. As the scale on each shutter reads from 
o to loo, any given color can be expressed by the percentage amounts of 
the standard colors used to reproduce the color : thus for a particular color, 
red 82, green 28, blue 63 was used. The color is thus expressed in definite 
terms and is reproducible. In the colorimeter one-half of the cylindrical 
lens is covered by an acute-angled prism which refracts the light from a 
fourth aperture through which the substance whose color is to be deter- 
mined is examined. The field of the colorimeter appears thus divided in 
two; half of it receives light from the colored substance while the other 
half is illumniated by the standard color plates. In using the instrument 
thi- field is first adjusted to a pure white and then the apertures of the 
standard color plates are opened until their resultant color matches that of 
the substance as it appears in the second half of the field. The standard 
color plates have been chosen with reference to their purity and their 
durability. 
Another recent device for the determination of colors is the chromoscope 
of L. Arons* which is based on the interference colors produced by quartz 
plates of known thickness on rotating the analyzer. A great range of tints 
and shades of practically every color can thus be obtained and definitely 
described by noting the thickness of the quartz plate and the angle in- 
cluded between the nicols, provided the same source of light be used for all 
determinations. 
Still another arrangement is being successfully developed by P. G.Nuttingf 
for this purpose. In his apparatus any given color is matched by adding 
to the proper spectral hue the quantity of white light necessary to produce 
the desired shade or tint. As both these quantities can be definitely deter- 
mined, the new method promises to furnish the most reliable values yet 
obtained. 
In petrographic microscopic work the lack of a suitable method for desig- 
nating colors properly has been keenly felt; but even now, after half a 
century of microscopic work, colors are designated in the same general terms 
that prevailed at the beginning. The color of one and the same mineral 
often varies in the thin section noticeably, and there is no doubt that, with 
a convenient and accurate method available, detailed studies of such color 
variations will lead to interesting conclusions regarding the effects of certain 
elements as pigments in crystal solutions. 
*Ann. Phjrsik (4). 33, 799-832 (1910). See also the Schistoscop of Brficke, Pogg. Ann., 74, 582 (1849). 
fOutline of Applied Optics. Philadelphia (in press). 
