Chromatic scale

“ If bodies where composed of thin layers such as those which form the chromatic scale and Newton's rings, their colours would change with every change of incidence, conformably to the law which we have just indicated. In nature the number of those colours that change is but small in comparison with those that remain fixed. ”

“ This system provides measured scales, established by special instruments, and is able to select the middle points of red, yellow, green, blue, and purple as a basis for comparing and relating all colors. These five middle colors form a Chromatic Tuning Fork. (See page 70.) It is far better that children should first become familiar with these tuned color intervals which are harmonious in themselves rather than begin by blundering among unrelated degrees of harsh and violent color. Who would think of teaching the musical scale with a piano out of tune? ”

“ Chroma is the strength of a color. It is the quality by which we distinguish a strong color from a weak one. The standard hue is approximately full chromatic intensity. Likewise each tint and shade is considered to be of its full chromatic intensity, making the left-hand boundary of the rectangle the area of full chroma. ”

“ Owing to the limitations of pigment-material, however, it is impossible to reproduce the intensities of the Spectrum satisfactorily. An approximation is all that we can achieve in painting.
THE SPECTRUM SEQUENCE AND
THE CIRCUIT OF THE COLORS 135. Having produced the scale of twelve colors in the values of their greatest intensities, and as intense as possible, we get an approximation to the Spectrum with this difference, that the color Violet-Red (Purple) which we get in pigments and mixtures of pigments does not occur in the Spectrum and, so far as we know, does not belong in the Spectrum. ”

“ Take as an example the sectors as set at two parts white, to one part black, the centre disc being vermilion, the slit is moved along the spectrum until such a point is reached that the colour reflected from the ring and the disc appears of the same brightness, for it must be recollected that they cannot differ in hue, as the light is monochromatic. It will be found that the place where they match in brightness is in the red, the exact position being fixed by the scale at the back of the slide. ”

“ All that can be expected from a physical theory is the determination of the composition of the light reflected from or transmitted by a thin plate in terms of the composition of the incident light. The further question of the chromatic character of the mixtures thus obtained belongs rather to physiological optics, and cannot be answered without a complete knowledge of the chromatic relations of the spectral colours themselves. ”

“ The test for a chrome is tinting strength, taking care to make a comparison with the same grade of colours, that is, light, medium or orange chrome. Fineness is another important test. Placing a small quantity on glass and passing a palette knife over it and pressing firmly will detect grit if present. In the lighter chromes it is well to look for the greyness of tone which is objectionable. Chromes mix well with white lead and are strong in body.
Ochres. —Analysis is of no value in determining the value of an ochre. Sometimes chrome yellow is used to tone it up. ”

“ In one respect the difficulty is greater in the case of the visible qualities, for there are no relations of colour which the eye peculiarly singles out and distinguishes, as the ear selects and distinguishes an octave or a fifth. Hence we are compelled to take an arbitrary scale; and we have to find one which is fixed, and which includes a proper collection of colours. The prismatic spectrum, or coloured image produced when a small beam of light passes obliquely through any transparent surface (as the surface of a prism of glass,) offers an obvious Standard as far as it is applicable. ”

“ In its very simplest form we have merely bands of color, or geometrical patterns made of squares, triangles, or hexagons. Here the artist has the maximum amount of freedom in the choice of color, the surfaces over which it is spread being of the same form and size, and hence of the same degree of importance. In such cases the chromatic composition depends entirely on the taste and fancy of the decorator, who is much less restricted in his selection than with surfaces which from the start are unequal in size, and hence vary in importance. ”

“ The concluding remarks appended to the chapter on yellow apply equally to red, and indeed to all other colours. It is not assumed that the list is exhausted: there are other reds, but they are, like some we have mentioned, ineligible as pigments, either by reason of their fugacity, their costliness, the difficulty of producing them on a scale, or the sources whence they are derived being commercially unavailable. While endeavouring throughout the work to render complete the collection of pigments actually in use, it is our object to give a selection only of numbered italicised colours ”

“ Science describes chroma as the purity of one wave length separated from all others. Other wave lengths, INTERMINGLING, make its chroma less pure. A beam of daylight can combine all wave lengths in such balance as to give the sensation of whiteness, because no single wave is in excess.8
(24) The color sphere (see Fig. 1) is a convenient model to illustrate these three qualities,—hue, value, and chroma,—and unite them by measured scales.
(25) The north pole of the color sphere is white, and the south pole black. Value or luminosity of colors ranges between these two extremes. ”

“ Upon this evidence, scales of red, yellow and blue were constructed of glass slips, each scale being all of one colour, with a regular variation of intensity from 0·01 to 20·0 units, equal units of the three scales being in equivalence with each other. The dimensions of the unit are necessarily arbitrary, but the scales comply with the essentials of a scientific standard, in that the divisions are equal, and the unit recoverable. The equality of the unit divisions in the scales, is demonstrated by a system of cross-checking. ”

“ Instead, however, of trying to effect a photometric match in any of the ways which have been found sufficient when the sources are of the same or nearly the same tint, we may effect important practical comparisons in what is called hetero chromatic photometry by an appeal to other physiological properties of the eye. For example, the power of clearly discriminating patterns in differently coloured lights of various intensities is obviously of great practical importance; and this power of detailed discrimination may be made the basis of a method of photometry. ”

“ Sir Boverton Redwood has used the scales and system for petroleum investigations. At his instance the specific colour rate of petroleum was established, and the several composing colours plotted in curves, as in Fig. 8, where the ordinates represent the scale of units irrespective of colour, and the abscissæ the scale of strata thicknesses.
The measurements were made at two-inch intervals, and the four perpendicular lines are at the colour points selected for valuing the four distinguishing marks, technically known as “Water White,” “Superfine,” “Prime,” and “Standard.” ”

“ Some of those changes are produced even by day if the colours of the scale are viewed through a green crystal, and others if they are viewed through yellow or azure crystals. Artificial light is without doubt differently constituted from that of the sun: it contains probably a scanty mixture of red rays with an abundance of the yellow, green, and azure. ”

“ Thus the red rays are least refracted, and the violet the most, and the yellow, green and blue are intermediate between them, being placed in the order [Pg 22] of least refrangibility. Between these there is of course every shade of simple colour, one melting into the other. In order to form a pure and bright spectrum with prisms, in a room of limited dimensions, we have to use certain auxiliary apparatus which are not positively essential, though convenient. The real essentials to form a spectrum are a narrow slit, a glass prism, with perfectly plane faces, and a lens. ”

“ Other measures fully bore out the fact that the luminosity of the mixed light is equal to the sum of the luminosities of its components. It is true that we have here only been dealing with spectrum colours, but we shall see when we come to deal with the mixture of colours reflected from pigments that the same law is universally true. ”

“ When a corundum is tested on the refractometer in white light a wide spectrum deliminates the two portions of the field because of the smallness of the colour-dispersion (cf p. 25) . The dichroism of both ruby and sapphire is marked, the twin colours given by the former being red and purplish-red, and by the latter blue and yellowish-blue, the second colour in each instance corresponding to the extraordinary ray. Tests with the dichroscope easily separate ruby and sapphire from any other red or blue stone. This character has an important bearing on the proper mode of cutting the stones. ”

“ We will now turn to a phase of colour measurement which will materially help us to see why, in some cases, the addition of white light to the simple spectrum colours, between the red and green, does not appear necessary in order to make a match with a mixture of red and green.
We will ask ourselves two questions: one is, whether any colour, and if so how much, can be added to white without appearing to the eye? ”

“ A third method of producing a chromatic passage—this time, however, with the left hand, without any assistance from the bow for the division of the notes—seems to be a speciality of Servais. It is introduced both in slow portamento effects and rapid chromatic passages. ”

“ How is it that the combination of three colours suffices to form white, or to match any colours we wish, be they spectrum colours to which a little white is added, or the colours of pigments? The most plausible theory that can be advanced is that it is only necessary for the eye to be furnished with a three-colour-perceiving apparatus to give the impression of every colour, and yet this would be somewhat difficult to believe had we not had the experiments narrated in that chapter before us. ”

“ Previous work with measured scales, made by the tuned crayons and tested by reference to the color sphere, have so trained the color judgment that children may now be trusted with more flexible material. They have memorized the equable degrees of color on the equator of the sphere, and found how lighter colors may balance darker colors, how small areas of stronger chroma may be balanced by larger masses of weaker chroma, and in general gained a disciplined color sense. Definite impressions and clear thinking have taken the place of guess-work and blundering. ”

“ It is a degree rather than a quality, as appears in the expressions, intense heat, light, sound,—intensity of stimulus and reaction. Being a degree of many qualities, it should not be used to describe the quality itself. The word becomes especially unfit when used to describe two very different phases of a color,—as its intense illumination, where the chroma is greatly weakened, and the strongest chroma which is found in a much lower value. “Purity” is also to be avoided in speaking of pigments, for not one of our pigments represents a single pure ray of the spectrum. ”

“ The spectrum, when recombined, should give a patch of the exact colour of that measured. The spectrum must be made narrow, as the template is only theoretically correct for a spectrum of the width of a line, as can be readily seen.
Templates like these will always enable any colour to be reproduced on the screen, and if the light be used for the spectrum in which the colour has to be viewed, be it sunlight, gaslight, starlight—whatever light it is—the colour obtained will be [Pg 106] that which the pigment would reflect if it were viewed in that light. ”

“ The mapping out is accomplished by means of a photographic scale fixed as a standard spectrum (Fig. 198) , in the position of A A, illuminated by 254 the small mirror at R, and focussed by a small lens at C, so that on looking into the instrument one can see the spectrum accurately divided into one hundred equal parts, and scale readings can be made at once; the only precaution needed is to be sure the D (or the sodium line, if D cannot be got) always stands at the same number on the scale. ”

“ Stains, as they occupy a comparatively limited area in the room color scheme, are of their full chroma value or reduced to three-fourths chroma. In only one instance (18) , Figure 463, do we find a reduction to one-fourth chroma, demanded by the nearly gray color scheme of the walls. We find it to be an established fact that small areas are capable of enrichment by colors of greater purity and higher chroma than larger surfaces. A silver pin may be designed to contain a stone of high brilliancy, but a wall surface has to be materially reduced in chroma to possess color harmony. ”

“ It is more instructive if we experiment with pure colours, and so we must resort to our colour patch [Pg 128] apparatus described in Fig. 6. If a small circular aperture about quarter of an inch in diameter be cut in a card, and placed in front of the prism nearest the camera lens (Fig. 32) , the colour patch, instead of being an image of the face of the prism, will be an image of the circular hole, and when the slit is passed through the spectrum we shall have a coloured spot on the screen, on which we can superpose a patch of white light from the reflected beam. ”

“ Children have made every example on this plate, with no other material than the five crayons of middle hue, tempered with gray and black. A Color Sphere is always kept in the room for reference, and five color balls to match the five middle hues are placed in the hands of the youngest pupils. Starting with these middle points in the scales of Value and Chroma, they learn to estimate rightly all lighter and darker values, all weaker and stronger chromas, and gradually build up a disciplined judgment of color. ”