Apparatus for Demonstrating Subjective Colours.
Year: 1896

page_text: Date of Application, 30th Nov., 1896-Accepted, 16th Dec., 1896
COMPLETE SPECIFICATION.
Apparatus for Demonstrating Subjective Colours.
I, SHELFORD BIDWELL, of Riverstone Lodge, Southfields, Wandsworth, in the
County of Surrey, a Fellow of the Royal Society, Barrister-at-law do hereby
declare the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:-
The invention relates to apparatus for the demonstration of certain colour
phenomena which are of physiological and not of physical origin, such phenomena
being termed subjective.
The essential part of the apparatus consists of a prepared disk of metal, cardboard
or other suitable substance, arranged so that it may be caused to rotate about its
centre.
The method of preparing the disk is as follows:-
(1.) An opening or gap is cut in the disk in some region lying between the central
portion and the circumference; such opening may extend to and include the actual
edge of the disk and may approach the centre as nearly as is consistent with the
stability of the central support. The most generally preferable form for the opening
is that of a radial sector having an angular aperture of from 45 to 60 degrees; but
I have obtained good results with openings in the form of circles and other figures.
The magnitude of the opening may also be varied within considerable limits.
(2.) One surface of the disk is painted black and white in such a manner that when
it is rotating these two hues are presented in alternation to an eye gazing in a fixed
direction upon the disk. The sequence of hues along any concentric circle, starting
from and ending with the opening or gap, should in order to obtain the best results
be either (a) Gap-black-white-gap; (b) Gap-white-black-gap; (c) Gap-
white-black-white-gap; or (d) Gap-black-white-black-gap. It should be
observed that the last mentioned sequence, (d), gives inferior results unless the
angular space occupied by one of the blacks is small, not exceeding ten degrees or
thereabouts.
It is sometimes advantageous, as, for example, when a slow rotation is desired, to
make two or more gaps in the disk instead of only one. In such cases the selected
sequence of black and white, along any concentric circle, should be repeated for each
additional gap introduced. Thus, if there were two gaps, the sequence (a) would
become First gap-black-white-second gap-black-white-first gap.
But in the absence of any special reason to the contrary I prefer to employ a single
gap.
The ratio of the total area occupied by black to that occupied by white may vary
considerably. In general I prefer to make the two areas about equal.
To make use of the apparatus the disk is placed so that a good light falls upon its
painted surface, which surface must face the observer. Behind the disk and close to
it is placed a piece of paper or other material upon which is a well defined design,
such as a geometrical figure, arabesque, picture, writing or printed characters. This
design will be seen at intervals through the opening in the rotating disk, and if the
speed of rotation is sufficiently high, the view of the object will, owing to the
persistence of visual impressions, be almost or quite continuous. Under these
circumstances the hues of the design and sometimes those of the ground will in
general appear to undergo certain changes or modifications.

page_text: In my opinion the best and most striking results are obtained when the design
consists of black lines or dots upon white ground, but I do not restrict myself to
the use of designs of this description. Thus the ground may be black and the
design white; or the ground and the design, or either of them, may be coloured.
In all such cases some change or modification of hue may result when the design is
viewed under proper conditions through the opening in the rotating disk.
If the effect is to be witnessed by an observer at a distance of some two or three
feet from the apparatus, it is preferable that the thickness of the lines constituting
the design should not exceed one thirty-second part of an inch or thereabouts, other-
wise the subjective colours will in some cases appear only upon the edges of the lines,
their inner portions retaining their original hue, or nearly so. If however the
apparatus is to be viewed from a greater distance, the thickness of the lines may be
advantageously increased in a degree proportional to the distance.
The following are two examples of the manner in which the invention herein
described may be best performed. For the sake of greater clearness reference is
made to the drawings accompanying this specification.
Fig. I represents a disk of white cardboard, six or eight inches in diameter, from
which a sector of sixty degrees is cut out, (a small portion being however left in the
central region for the sake of mechanical strength). The remainder of the disk is
divided into two equal parts by a straight line extending from the centre to the
circumference, and one of these parts is painted black. The disk is to be mounted
at its centre upon an axle or pin in such a manner that it may be capable of rotation.
The rotation should generally be at the rate of not less than five turns per second,
and may be imparted to the disk in any suitable manner, as by clock work, or by
hand-driven gear, or simply by striking the edge tangentially with the finger.
Let a white card upon which the letter A (or some other design) has been drawn
in black lines one thirty-second part of an inch thick (or thereabouts) be placed behind
the disk, as indicated by dotted lines in the figure, and let the disk be caused to
rotate in the direction shown by the arrow, while its front surface is exposed to
illumination. The design as seen through the opening will at once lose its blackness
and assume some other hue, the nature of which will depend partly upon the quality
and intensity of the illumination, partly upon the speed of rotation and partly upon
the idiosyncrasy of the observer. If the disk is placed at a distance of one foot (or
thereabouts) from a lamp of about twenty standard-candle power; if also it makes
about five or six turns per second and if the observer's vision is normal, the design
will appear to be of a bright red hue; while if the direction of rotation is reversed, the
design will become blue.
Fig. II gives an example of a disk in which the black portion is bounded by curves.
The sequence of black and white along circles concentric with the disk and in the
direction of the arrow corresponds to that hereinbefore distinguished as (a) in
the inner region of the disk, to (4) in the outer and to (c) in the intermediate
regions. This disk when used under the conditions before specified will impart to
the black design a series of colours roughly approximating to those of the rainbow.
The above examples sufficiently indicate what I believe to be the best arrangement
of the apparatus, though endless variety in details is possible.
•
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
A rotary disk having a black and white surface and one or more openings, for
the demonstration of subjective colour phenomena in a fixed design, substantially
as herein before described.
Dated this Twenty-eighth day of November 1896.
SHELFORD BIDWELL.