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red rays from an electric arc, the luminous rays being removed by transmission through a solution of iodine, on a strip of platinum foil, when the platinum was heated to incandescence and emitted visible radiation.

392. Chemical Action.-When light is absorbed by a body the energy of the absorbed radiation is taken up by the body, and we have already considered some of the forms under which this absorbed energy can exist, namely, it can be converted into heat and warm the body, or it can produce by fluorescence or phosphorescence light of different character from the incident radiation, and be again radiated as a vibratory motion. We have now to consider a third way in which the energy of absorbed light may be used, namely, in doing chemical work. Thus a mixture of chlorine and hydrogen gases will keep indefinitely in the dark, but as soon as the mixture is exposed to sunlight the two gases combine with explosive violence.

Another well-known case where light produces chemical change is that of silver chloride, which, under the influence of light (sunlight), becomes blackened owing to the reduction of the silver. This provocation of chemical change by light is, of course, the cause of all photographic action.

Light of all colours is not equally active in promoting chemical change, and in Fig. 373 the relative intensity with which light of different colours

ABC D E F

T

HJM NOR STUV

FIG. 373.

of the solar spectrum act in promoting chemical change is shown by means of a curve. The exact position of the maximum, however, depends to a certain extent on the nature of the chemical change produced. The spectrum must be produced by means of a quartz prism and lenses, for glass exerts a powerful absorption on the ultra-violet rays. It will be seen that the rays that are chiefly efficacious are those in the extreme violet and in the ultra-violet. For this reason the ultra-violet rays are often called chemical rays, but it must be remembered they only differ from the visible and heat rays in their wave-length, and that chemical action is not confined to these rays, but is only more strongly exhibited by them than by rays of greater wave-length,

393. Extent of the Light and Heat Spectrum.-It is of some interest to briefly collect a few data as to the range of wave-lengths which have been measured in the case of light or heat radiation.

In the following table the approximate wave-lengths of a few interesting kinds of radiation are given :—

Smallest wave-length measured

Maximum of chemical action in solar spectrum. Ex

tremity of visible spectrum

D line.

Extremity of visible spectrum

Cm.

10000'

.00004

.000059

Maximum energy in solar spectrum

Largest wave-length measured

Smallest measured electrical oscillation

.00007

.00008

.0025
.6

The last number of the above table has been added on account of the fact that, on the electro-magnetic theory of light, both light and heat waves are really electrical oscillations (§ 581) of small wave-length, and the numbers given show that there is not such a very great breach to be filled up before we have a series of measured wave-lengths, i.e. a spectrum, extending continuously from the ultra-violet down to electrical oscillations which are observed as such.

CHAPTER IX

COLOUR SENSATIONS

394. Sensations produced by Light.-We have up to the present considered the subject of light in its objective aspect only, and must now proceed to examine the subjective sensations produced when light-waves of various kinds enter the eye. When dealing with the subject of audition, we saw that the nature of the sensation produced depends on the intensity, the frequency, and the timbre of the note. The timbre, however, is really included in the first two, for it depends on the frequency and intensity of the various simple tones which build up the note. In the same way, the sensation experienced when light enters the eye depends on the frequency and intensity of the various simple coloured lights which build up the resultant colour. As we shall, however, find, the eye possesses much less analysing power than the ear, for while the ear which receives a note of given composition can always distinguish any other note of which the composition is different, this is not the case with the eye. Thus by allowing light of only three selected frequencies to enter the eye, a sensation is produced which is quite undistinguishable from the sensation produced when white light enters the eye, although, as we have seen, white light consists of light of all frequencies between very considerable limits. Thus while in acoustics like sensations are produced by like causes, in optics this is not necessarily true, and the same sensation may be produced by entirely different causes.

395. Colour Constants. We shall in the following pages use the word colour in a rather different sense to that hitherto employed. Up to now, by the colour of light we have meant the frequency of the ether vibrations, and so have used it in the sense of pitch in acoustics. Now, however, we shall use the word colour to designate the sensation produced in the eye, although where confusion is likely to occur the expression colour sensation will be employed.

As we shall see later, the colour sensation produced in different persons by the same quality of light may vary considerably, and so we have to consider the sensation which is felt by the majority of people; in other words, we shall deal with the normal eye.

In order to specify a colour it is necessary to know three things about it. In the first place, we require to know the frequency of the various vibratory motions which constitute the light which enters the eye, or, as

it is sometimes called, the hue of the light. In the second place, we require to know the brightness or luminosity of the colour. In the third place, we require to know whether the light considered is mixed with any white light, and if so, to what extent. If a light is free from admixed white light it is said to be pure. Thus by allowing monochromatic D-light to fall on a white card, the sensation is that of orange-yellow and the colour is pure. If, however, the card is simultaneously illuminated by white light, the sensation produced is altered and the colour is no longer pure.

396. Luminosity. In order to be able to measure the luminosity of a colour, we must have a standard or unit of luminosity. Two cases have to be considered, namely, when we are dealing with the luminosities of different coloured lights, and when we are dealing with the luminosities of the colours seen when different pigments are illuminated by white light.

When dealing with lights of different colours, the unit taken is some of the white light produced by the source which is employed to give the coloured light. In the case of pigments, the unit is the luminosity of a white surface which is illuminated by the same light as that which falls on the pigment.

As a source of white light which may be employed in colour measurements, Captain Ab

ney has found that the light given by the crater of the electric arc (496) is by far the most steady and uniform in quality. The arrangement he has employed in his experiments on colour is shown in plan in Fig. 374. An image of the crater of an arc, E, is thrown, by means of a lens, L1, on the slit of a collimator. The parallel beam of light thus produced falls on the prisms P1 and P and is thus split up, the different coloured rays being brought

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to a focus by the lens

Lg between V and R. If a screen is placed at A, a pure spectrum will be

formed on it. If the screen is not there, the lens L3 will cause the light of all the various colours to be superposed over a small patch on a screen at B, and so will reproduce white light. If, however, there is a screen at å in which there are one or more slits of which the positions can be varied, then it is only the light of the wave-lengths corresponding to the positions of these slits in the spectrum which will be thrown by the lens L on the patch B. Hence by varying the positions and sizes of these slits, different colour mixtures can be obtained. Some of the white light is reflected from the first prism along R, and by means of a lens and a mirror this light is caused to form a white patch on the screen at C, and this acts as a reference white when measuring luminosities. The intensity of the white light can be reduced by means of a set of rotating sectors placed at D, which are so arranged that the proportion of opaque sector to transparent sector can be adjusted while the instrument is rotating.

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In order to determine the luminosity of the different parts of the arclight spectrum a single slit is employed, so that the patch B is illuminated by light of one wave-length, and the intensity of the white light is varied till the two patches appear of equal brightness. By making this comparison all along the spectrum, and plotting the luminosities obtained as ordinates, Abney has obtained the curve shown in Fig. 375. The fullline curve represents the luminosity for a normal eye, and it will be seen that there is a very marked maximum in the yellow, and that the luminosity of the violet end of the spectrum is very small. The dotted curve represents the luminosity as measured by a red colour-blind observer, and we shall return to this subject later.

If the luminosity of two coloured lights is measured separately by

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