Appendix Iii

THE APPLICATION OF THE NATURAL LAW OF SPECIFIC COLOUR RATE BY DR. DUDLEY CORBETT TO THE EXACT MEASUREMENT OF X-RAY DOSAGE.

Dr. Dudley Corbett.

The gradations in the tint given by the Sabouraud-Noiré pastille when exposed to X-rays are so fine, especially in that region of the colour scale where lies the erythema dose, that many have felt the want of a more accurate means of reading these tints, as well as a series of reliable standards for comparison. Hitherto the only methods at all generally used have been Hampson’s radiometer and Bordier’s radio-chronometer, the former in this country, and the latter on the Continent. Hampson’s instrument has two disadvantages: it can only be used with electric light, and the standards are made of tinted paper liable to get soiled, and to vary slightly with the changes in the pigment employed. Its advantage is that it may be used as a sliding scale, thus economising the pastilles. Some persons, however, have considerable difficulty in reading the tints, the scale rising only by gradations of 1/4 B.

In the construction of any such instrument, the really important point is to obtain a reliable standard for Tint B--i.e., the normal epilation dose. The tints on the Sabouraud card itself are not always identical, some representing a dose which will only just epilate, others an almost dangerous dose for unfiltered rays. The Tint B, which is the standard, allows a margin of error of 20 per cent. on either side. In other words, 4/5 B will almost always epilate, while 1-1/5 B is nearly the limit of safety. This observation is in accordance with the experience of other workers on this subject. In my instrument Tint B has been obtained by measuring the pastille with Lovibond’s tintometer immediately after exposure to the X-rays. The pastille was turned to a tint corresponding to an epilation dose which was known to be safe, as proved by clinical results. This tint was measured directly both by daylight and by artificial light from an 8-candle power carbon filament lamp with frosted glass shade. I am indebted to Mr. Dean for suggesting the use of Lovibond’s instrument for this purpose.

The methods employed in the experimental work have been described in the British Journal of Dermatology for August, 1913. By using a very constant focus tube and averaging a large number of readings and correlating the results with those obtained in clinical practice, we were able to construct the curves indicating the colour developed by the pastille. In these curves, shown in Fig. 10, the ordinates are the Lovibond colour units, the abscissa the time during which the current was actually passing. When using an interrupter working at a constant speed, the actual time was taken, otherwise the number of current interruptions as measured by a dipper tachymeter was used.

As was to be expected, the daylight and electric light curves were quite different. In each case the standard yellow glasses employed were kept constant throughout the curve, that for daylight being 15 units, that for electric light 13 units. When these were combined with blue and red glasses in varying units and fractions of a unit, they gave a colour range which matched the pastille exactly in the changes it undergoes from the unexposed condition to the 2 B Tint.

The curves are plotted in accordance with Mr. Lovibond’s practice--that is, not as a direct representation of the standard glasses used, but as showing the colour sensation received by the eye.

First, as to the daylight curve: In order to match the unexposed pastille we interpose between the pastille and the observer’s eye a yellow and a blue glass, plus a certain amount of neutral tint (composed of three equal colour units). Thus the colour sensation received is a yellow green, together with a certain amount of white light. As the pastille darkens under irradiation, both the green and the white light disappear, until at a point just below the 1/2 B dose, there is no other colour present but yellow. After this red glasses are required--i.e., the colour sensation is a yellow orange, which gradually deepens owing to an increase in the proportion of red. The yellow curve thus rises till just below the 1/2 B point, and then falls as the orange increases.

Next, as to the electric light curve. The unexposed pastille has but a trace of green, which is soon lost. The orange begins much earlier than in daylight, and thus at Tint B has reached a higher point than in daylight. From this point the orange and yellow parts of both curves run practically parallel with one another up to 2 B. Beyond 2 B the readings become more difficult. I have not determined the point when no more colour develops, as it has no great practical value, though it might well be of interest from a physico-chemical standpoint.

In my radiometer the standards are composed of the Lovibond standard glasses in combination. The apparatus itself consists of an optical instrument or viewing box. This is divided by a central partition, so that on looking through the eyepiece one sees a white background through two small circular apertures. On one side, level with the background, is a fitting to take the pastille in its holder. On the other side is a groove in the instrument itself for the insertion of the standard glasses. A similar groove is fitted on the pastille side of the instrument to take neutral tints if required. The colour of the pastille as seen by reflected light can thus be compared with that obtained by transmitted light through the standard glasses seen against the white background. A difference of 1/5 B or 1 H is quite easily perceivable.

It is usually of no great importance to obtain extremely accurate measurement of the smaller fractions below 1/3 B. Where this is necessary, neutral tints must be used when working with daylight. With electric light these are unnecessary. When required the neutral tints are interposed between the pastille and the eye to absorb the white light reflected from the pastille. The neutral glasses required are 1·5 for the unexposed pastille, 0·6 for 1/4 B, and 0·2 for 1/3 B. These values are subject to slight variations due to changes in the varnish of the pastille emulsion. The difficulty can always be avoided by using electric light, where a trace of neutral tint is needed only when matching the unexposed pastille--an unimportant point.

Method of Use.--The choice of daylight or artificial light is a personal matter, but one should practise reading the scale with both. The use of the instrument shows that the pastille fades very nearly as quickly under electric light as it does under daylight. The following precautions should be observed: In daylight work in a good white light, avoid shadows and yellow light of any kind. With electric light use an 8-candle power carbon-filament lamp with frosted glass and a suitable white shade so arranged that the pastille is 8 inches from the lamp. No other light should be allowed to reach the pastille during examination. A low power metal-filament lamp may be used, but greater accuracy will be obtained with a carbon-filament lamp which was used for the experimental work. The lamp should be discarded as soon as the light becomes yellow from prolonged use. Whether in daylight or electric light, the examination must be rapid to avoid the fading of the pastille. When it is desired to give an accurate 1 B dose, it is better to put up the 4/5 B standard first. It is then easy to calculate how much more exposure is required for the extra 1/5 B. It is important to adjust the pastille carefully so that none of the unirradiated green portion is visible through the small aperture, as this will upset the reading. In very accurate dosage new pastilles should be used, as a bleached pastille never returns exactly to its original tint. When such a bleached pastille is irradiated the colour changes start a little farther down the curve, and thus the tint for a given dose must be taken a little above the normal tint. This increase is very slight, but is nevertheless quite appreciable, and may amount to as much as 5 and 10 per cent. Even then the margin for error is ample in the neighbourhood of the B tint, and if a pastille is not used more than three times, and is well bleached in daylight after each exposure, no serious error is likely to occur. A standard white background should always be used, and discarded for a new one when it gets dirty. The colour standards usually provided are those in common use--namely, 1/4, 1/3, 1/2, 4/5, 1, 1', and 2 B, but it is quite easy to make up standards for any point on the curve. The symbol “B,” as the erythema or epilation dose, has been retained, as it was thought inadvisable to add to the number of such symbols already existing.

To sum up:

1. The experimental work has determined the exact colour changes occurring in the Sabouraud pastille when exposed to X-rays.

2. These experiments have established a permanent standard for Tint B, which matches the pastille exactly, does not fade, is easily kept clean. These coloured glasses can be readily and accurately reproduced, as they are standardized spectroscopically by a firm who specialise in such work. The standard will therefore remain constant so long as the Sabouraud emulsion remains unaltered.

3. Glasses may be prepared of the correct tint for any fraction or multiple of this dose up to 2 B or 10 H.

4. Either daylight or electric light can be used.

5. The optical instrument itself, by cutting off extraneous light, greatly assists the colour comparisons, so that the practical error need never exceed 10 per cent.

INDEX

Abnormal light, 13

Analysis of white light, 11, 13

Arbitrary scales, 9

Artists and scientists, 1, 2

Beam of white light, 7, 11

Black units, 16, 23

Black, ideal, 15, 11

Blood curves, 33

Code of laws, 7

Colour charts, 31, 39 education, 59 equivalence, 10 nomenclature, 17, 21 qualitative, 17, 19, 24 quantitative, 24 scales, 20, 29 standardization, 20, 69 theories, 2, 3

Coloured surfaces, 44

Colours, complex, 18 sadder than standards, 25 simple, 17

Corbett, Dr. Dudley’s, radiometer, 83

Daylight colours, 24, 84 measurement, 14

Diffused light, 10, 43

Direct lights, 46

Dulled colour, 10, 18-26, 36

Equivalent colour units, 7, 9, 29

Fog, whiteness of, 10

Glass standards, 20

Human blood curves, 82

Laws of colour, 7

Light, 11, 13, 42, 45-46, 81-84 abnormal, 8 brighter than standards, 14, 27 direct, 46 for colour work, 42 intensities, 42, 43 white, 14

Lovibond’s new colour theory, 5

Matching colours brighter than standards, 18 complex colours, 17

Measuring and naming colours, 21

Monochromes, 17, 19

Munro, Dr., 34

Neutral tint, 18, 24, 26

North light, 15

Past theories, 3

Photographic energies, 53

Physical colour constants, 8

Pigmentary black, 15, 71

Primary colours, 1

Prismatic spectrum colours, 29

Qualitative analysis of colour, 17, 19, 24

Quantitative analysis of colour, 24

Radiometer, 83

Rate of colour absorption, 8

Rays, six colour, 7

Red ray, 41

Scales, arbitrary colour, 9 coloured, 10, 20 cross checking of, 9

Scientists and artists, 1, 2

Sea fog, 10

Specific colour, 32

Spectrum colours, 4, 18, 36

Three colours, 10, 57

Time, appreciation of colour by, 42

Ultra violet, 32, 40

Unit, checking of, 9

Unit, neutral tint, 18, 24, 26

Wave length position, 37, 38

White light, 11, 13

Butler & Tanner Frome and London

Transcriber’s notes:

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