electrodes cannot be obtained commercially, and the illuminating engineer has to fall back practically upon screens for obtaining colored effects.

Color in lighting may be utilized to intensify the hue of objects already colored or to impart color to things not already possessing it. Light as nearly white as possible brings out the natural color values in a fairly uniform way. A single color gains in brilliancy from flooding with the same color, while illumination with the wrong color may utterly spoil the effect. These things are, of course, perfectly familiar in interior lighting. The decorative value of color has been comparatively little appreciated or utilized in exterior illumination. The most striking instance of its employment on a large scale was at the Panama-Pacific International Exposition of last year, at San Francisco, in which for both day and night effects color played a predominant part. In regular flood lighting work a monument or even a sign may be so tinted as to gain from the application of a particular color in its illumination. But instances where this can be advantageously applied are rather rare.

Perhaps of more general importance is the possibility of producing highly decorative results in the illumination of façades of buildings by giving them color values which relieve the monotony of the effect otherwise attainable. Comparatively little has been done in this line, although the writer tried it out experimentally on the façade of the Massachusetts State House and of the building of the Edison Illuminating Company last year far enough to learn something of its possibilities. The chief difficulty in such work, which can be carried out with very beautiful effects, is to obtain the necessary illumination without too great cost in energy. Screens of the colored film used in theatrical work can readily be arranged in conjunction with lamps for flood lighting. In the case of the experiments just referred to the screens were fitted into frames in racks just in front of the lamps. In theatrical working the areas to be covered are small and the available intensities are so great that a considerable range of color can be successfully employed. This range is much limited in the larger problems of exterior lighting unless at great cost of energy. Light yellow screens fail to produce any striking effect. Even amber tints, although losing considerable light, do not seem to produce a good hue on the surface illuminated. Light reds work better and light rose pinks also are very successful. Greens and blues are not very striking unless deep in color and consequently wasting much light.

In general terms the loss of light in colored screens of hue deep enough to produce any material effect is from 50 to 80 per cent. so that one has to allow from 3 to 4 or 5 times the intensities which would ordinarily be utilized for flood lighting. It is not necessary to fit all the reflectors with screens in doing such work. A ground illumination can be produced in the oridinary way and then tints laid on by banks of special reflectors directed either so as to overlay the whole or any part of it with warm color.

Considerable experimenting is needed to produce screens which will give the maximum of tinting effect with minimum loss of light and which will retain their color without fading. Of course, the films used for theatrical purposes will not withstand moisture so that when used out of doors they must either be screened in with glass or withdrawn in rainy weather. The colored applications are interesting, and probably will be made an important adjunct in flood lighting, but the whole matter is still in the experimental stage.

MODERN PHOTOMETRY

BY CLAYTON H. SHARP

The present lecture is to be looked upon as in a measure a continuation of the lectures on the measurement of light given in the 1910 I. E. S. course at Johns-Hopkins University. It is intended to supplement those lectures not only by introducing an account of the developments in photometry since 1910, but also by treating of certain matters which were either insufficiently treated or were omitted entirely from the 1910 lectures. It should be understood, however, that it is the intention of the lecturer not to attempt a complete review of photometric advance during recent years, but rather to confine himself to the practical features which properly belong in this essentially practical course.

The practice of to-day in the measurement of light involves innovations and improvements which the change of conditions since 1910 has brought forward. Since 1910 the introduction of the gas-filled tungsten filament incandescent lamp with its whiter light has made the photometric difficulties due to color differences a more important factor in the art and has been a direct incentive toward the prosecution of the investigation of the problem of heterochromatic photometry and of the introduction of means to solve it, while the increasing demand for accuracy in photometric measurements, and particularly the growth of the idea of the measurement of luminous output of all lamps in terms of their total luminous flux rather than in terms of their candle-power, has given a great incentive to the use of the integrating sphere. During the six-year interval new and improved types of apparatus have been constructed and put into

use.

PHYSICAL PHOTOMETER

The physical photometer, an apparatus which will measure the light from any illuminant and give the result in terms identical with those which would be obtained by the use of a photometer by a person of normal color vision, has been realized. This physical photometer has been constructed and practically used by Ives1 who uses a sensitive thermopile as a means for measuring the radiant energy. He has two methods for selecting the radiation from the

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lamp in accordance with the luminosity curve of the average human eye. The first of these methods involves passing the light through a spectroscope equipped with a shield or screen which is cut out in the form of the luminosity curve. The spectrum, which is thereby reduced to a luminosity curve spectrum, is reunited, and the total energy passing through the screen, which is then proportional to the light of the lamp,. is thrown on the thermopile. The second method, which for experimental purposes is undoubtedly simpler, involves passing the light through a glass cell having a thickness of one centimeter containing the following solution:

Cupric chloride...

Potassium ammonium sulphate..

Potassium chromate..

Nitric acid, gravity 1.05...

Water added to make one liter.

60.0 grams

14.5 grams

1.9 grams

18.0 c.c.

Between the solution and the lamp is interposed another water cell to prevent overheating of the solution. The transmission of this solution is according to Ives identical with the luminosity curve of the average eye.

FLICKER PHOTOMETER

Ives' has recommended a system of heterochromatic photometry involving the use of a standardized form of flicker photometer and the investigation of the color vision of the observers using it. The flicker photometer as recommended by him has a field two degrees in diameter with a surrounding field of large dimensions illuminated to approximately the same degree. As the standard illumination for the flicker field he recommends 25 meter-candles.

A simple attachment to be used on an ordinary Lummer-Brodhun photometer to convert it to a flicker photometer corresponding to these specifications has been described by Kingsbury2 and is expected shortly to be commercially available. Ives has shown both theoretically and experimentally that the settings of observers using a flicker photometer are affected by peculiarities of their color vision. He has, therefore, proposed a criterion for normality of color vision of observers using the flicker photometer. This consists in measuring the light of a 4-wpc. carbon lamp through a one centimeter layer of each of two different solutions. The first consists of 72 grams of potassium bichromate in water to make one liter. The trans1 Ives, I. E. S. Transactions, 1915. page 315. A bibliography of the subject is there given. 2 Kingsbury, Journal of Franklin Institute, August, 1915.

mitted light with this solution is yellowish. The other solution consists of 53 grams of cupric sulphate in water to make one liter. This gives a bluish color. The solutions are to be used at 20°C. Ives has shown that a person with perfectly normal color vision will find with a flicker photometer the same value for a 4-wpc. lamp with either solution. His proposal then is to make color measurements using the flicker photometer and a group of observers so selected that on the average their value for the transmission of the yellow solution is the same as that of the blue solution, such a group having according to his measurements normal color vision. This proposal has been thoroughly investigated by Crittenden and Richtmyer3 who by studying the peculiarities of a large number of observers using a Lummer-Brodhun photometer have shown that identical photometric results are obtained by a selected small number of observers having on the average normal color vision as determined by Ives' criterion and using a flicker photometer.

CROVA'S METHOD

Ives' has shown that an incandescent gas mantle can be compared without error with a 4-wpc. carbon lamp using an ordinary photometer and interposing between the eye and the photometer a 25 mm. layer of the first of the following solutions. To effect a comparison between a 4-wpc. carbon lamp and other incandescent electric illuminants the second of the following solutions is used:

When using the first solution with a mantle burner against a 4.85-w.p.scp. carbon standard, the standard has a value which is one divided by 1.065 times its true value. No correction is necessary in using the second solution. The use of this solution has the great advantage of eliminating not only the color difference between the lights as seen in the photometer field, but also the effects of pecu

Crittenden and Richtmyer, I. E. S. Transactions, vol. 11, page 331, 1916.

Ives, Physical Review, page 716, 1915. Ives and Kingsbury, I. E. S. Transactions, vol. 10, page 716, 1915.