extensively used for general lighting in many cases where the requirements are not as refined as in the previous cases, and where eyes untrained in refined color-discrimination are involved. Among the places and occupations in which this quality of daylight is at present being used are the following: lithographing, paint shops, and stores, tailor shops, wall-paper stores, in green houses, artists' studios, art galleries, operating rooms in hospitals, paper mills, flour mills, garment factories, shoe stores, textile mills, florist shops, dry cleaning, laundries, furniture stores, undertaking, millinery shops, haberdasheries, art schools, and in illuminating color photographs. Approximate Artificial Daylight.-Under this head will be discussed briefly the applications that have been made of "artificial daylight" which is only an approximation to average daylight being in reality a compromise between quality and efficiency. This discussion of the application of this quality of light, which in these particular cases is obtained from a tungsten lamp corrected by means of a colored glass bulb so that its visible spectrum closely approximates that of a black body operating at a temperature midway between the melting point of tungsten and the apparent temperature of the sun, applies to any other artificial light-source similarly corrected. The efficiency of light production has reached a point where it has proved expedient to obtain a better quality of light by sacrificing some of the light. Data are available from installations involving many thousands of lamps but only a few points will be discussed for the purpose of showing the trend in this aspect of artificial daylighting. A record of the applications of this approximate daylight includes all of the fields included under the preceding two paragraphs with the exception of the cases where very accurate color discrimination is required. Investigation shows that such a quality of light is in use for general lighting in the following places and occupations and perhaps in others: department stores, haberdasheries, cigar stores, art galleries, clothing stores, millinery shops, tailor shops, shoe stores, jewelry shops, paint and wall paper stores, furniture stores, undertaking, laundries, dry cleaning, medicine and surgery, hospitals, color printing, hardware stores, libraries, grist mills, florist shops, automobile display rooms, textile plants, illumination of color photographs, photographic studios, offices, drug stores, hair goods shops, stationary stores, barbor shops, laboratories, microscopy, grocery stores, confectionary stores, upholstering shops, breweries, hair dressing, show windows, fur stores, and in a number of isolated places. The application of this illuminant to art museums is especially worthy

of attention owing to the exacting requirements. A number of museums are at present equipped with this illuminant, notably the Cleveland Museum of Art. One interesting result has been the popularity of the museum at night. These applications of artificial daylight are sufficiently numerous and. diversified to indicate that consumers are not universally satisfied to accept the accidental quality of light emitted by various artificial illuminants providing a much better quality of light can be obtained without a prohibitive loss in luminous efficiency. This is a natural result of the education of the public resulting from the activities of this society.

No further discussion of the applications of artificial daylight appears necessary in those fields which prominently involve the appearance of colors; however, artificial daylight has found its way into fields not generally expected. For instance, there has always existed a feeling of unsatisfactoriness in the lighting during the period of the day when daylight must be reinforced by artificial light. This is perhaps partially due to a difference in the distribution of light in the two cases. However, the difficulty is also partially, if not largely, due to the difference in color. Experiments with artificial daylight for desk-lighting have been quite convincing to many persons. A number of installations of approximate artificial daylight units have indicated that this is probably a large field for future development. Physiological and psychological research has yet to explore this field. Many other unique applications could be discussed to advantage but it is believed that sufficient space has been given to this subject at present. However, it has been considered profitable in this lecture to devote considerable space to this development in lighting because it represents perhaps the stride of greatest magnitude and portend in the application of the science of color in lighting that has been made recently.

Applications of Color Mixture.-Many diversified applications of the principles of color mixture are open to the lighting expert. The stage offers the greatest possibilities although ordinary specifications of stage-lighting often provide only clear, red, and blue lamps. It is obvious that the range of colors resulting from mixtures of these is quite limited. When it is considered that the lighting effects are valuable tools in the hands of the stage director it is wondered why facilities are not provided for using at least the three primary colors, red, green, and blue, and also clear lamps. If space permits it would be desirable to add yellow lamps. Of course, yellow could be obtained by mixing red and green but inasmuch as it is an important

stage-lighting color it appears undesirable to sacrifice it in obtaining the red and green originally and then to produce it again by mixture at a greatly reduced efficiency.

The primary colors have been used in show windows and for many special effects. One unique installation is found in a pretentious residence. Red, green, and blue lamps are installed above a large oval panel of opal glass set in the ceiling of a dining room. Any quality of light could be obtained by controlling various lamps by means of three rheostats located in a cabinet in the wall. A number of installations on a larger scale have been placed in ballrooms and restaurants. Such applications should be more numerous considering the pleasure obtainable. A few cases have been noted where colored lights have been mixed for the general illumination of theatres, bill boards, special displays, ball rooms, etc. Flashers have usually been used but rheostats can be readily designed to be mechanically operated so as to vary the intensity of the various components by imperceptible increments. Beautiful effects have been obtained by illuminating clothing models with mixtures of the primary colors, accentuating the effects occasionally by directed unaltered light. The latter effect is intensely beautified by the colored shadows which remain due to a flood of colored light of a lower intensity than the clear directed light. Incidentally this brings out the point that colored shadows can be used in many lighting effects with wonderful success. Many possibilities of the use of color in lighting are found in interiors. Colored lights obtained by mixture provide pleasing variety and deal harshly with the monotony of ordinary lighting installations. In ordinary lighting tints are more satisfying to the æsthetic sense than saturated colors and these tints are readily obtained by adding lights, fairly saturated in color, to the ordinary unaltered light. In general it is necessary to conceal the sources. In the home the tint can easily be adapted to fit the place, the occasion, or the mood. Various possibilities can be provided in different rooms or in the same room. Moonlight, sunlight, candle-light, fire-light, etc., can be provided with ease.

Recently a moving picture theatre has been provided with a yellowish light of low intensity for use ordinarily during the projection of pictures and a bluish light for use when night scenes are on the screen. This is an example of the many possibilities of using colored light in illusory presentations.

Colored light has been used successfully in the flood-lighting of monuments, buildings, and pageants.

Special Color Effects.-In a few rare instances colored light has been applied to billboards and other displays and doubtless this field for colored light will be developed eventually. The play of colored light on properly painted displays is attractive and when the efficiency of light production has sufficiently increased these applications should increase in number. Special color effects have been proposed in which complete changes are produced by properly associating the colored pigments used in painting the scene, or advertising material, with the colored illuminants. These should eventually find a wide field on the stage and in displays. A few applications have been made but the difficulty at present lies in the necessity of a complete grasp of color science in order to accomplish the desired results.

Notable Installations of Colored Light.-A notable installation of luminants of different color and brilliancy is found in the Allegheny County Soldier's Memorial Building, Pittsburgh, in which mercury arc, Moore tube, flame arc, and tungsten lamps are woven into harmonious effect.

The applications of light and color at the Panama-Pacific Exposition are well known. This installation represents one of the greatest undertakings in lighting ever attempted and also stands as an example of the achievements that can be attained by the lighting expert who has the hearty coöperation of architects and other responsible authorities.

Simulating Old Illuminants.-A few applications of this character have been made but it is difficult to discuss this subject analytically because the requirements are not sufficiently exacting to demand uniformity in the developments. The results have been obtained by the use of color in ornamental glassware, of colored screens over the aperture of indirect units, of colored fabrics, and of colored lamps. Many interior lighting units approach this result by the unconscious application of warm tints to the lighting. accessories. In those cases where the aim has been specifically to simulate older illuminants a common error has been made in employing an amber color instead of an unsaturated yellow as discussed earlier in this text.

Modifying Daylight.-A few installations of this character have been noted, the object usually being to eliminate the cold appearance of daylight by using ceiling or side windows glazed with an unsaturated yellow glass. Several notable installations are found in pretentious buildings. A satisfactory glass has been obtainable in the market. Such applications have their best field in open

ings where only skylight enters. A specific instance was observed in an, elaborate hotel where a ceiling window at the bottom of a lighting court was glazed with a yellowish glass. Other instances have been found in residences. In one case the windows in the dining room received little sunlight and the windows were glazed with a transparent yellow glass. The effect of the unobtrusive, unsaturated yellow glass was always pleasing and extremely so on dismal rainy days. Stained glass windows are colored chiefly for decorative effect but the modification of the light which passes through them often adds variety and interest to the interior.

Bibliography

In this general lecture it has been thought best to exclude references to various investigators and practitioners who have contributed to the progress of the art because historical treatment would lead the discussion far afield; however, a bibliography of the representative work on the subject has been appended. No pretense to completeness in the bibliography is entertained, although the following references have been selected with this lecture in mind. Preference has been given to published work of recent years, to those works which include extensive bibliographies of the available material, to discussions treated from practical viewpoints, and to the availability of the publication. As a result of such a procedure and of the desire to be concise, many worthy papers have not been directly mentioned. However, by referring to the various bibliographies found in the publications actually referred to, a comprehensive view of the various subjects can be obtained.

J. W. BAIRD. "Color Sensitivity of the Peripheral Retina." Carnegie Inst. Pub. 1905, p. 80.

PAUL F. BAUDER.-"Reflection Coefficients." Trans. I. E. S., 6, 1911, p. 85. LOUIS BELL.-"Monochromatic Light and Visual Acuity." Elec. World, 57, 1911, p. 1163.

E. J. G. BRADFORD.-"Color Appreciation." Amer. Jour. of Psych. 24,

1913, p. 545.

E. J. BRADY.-"Daylight Glass." Trans. I. E. S., 9, 1914, p. 937. BROCA and SULZER.-"Growth and Decay of Color Sensations." Comp. Rend. 2, 1902, p. 977, p. 1046.

M. E. CHEVREAL.-"Harmony and Contrast of Colors," 1835.

J. COHN.-"Gefühlston und Sättigung der Farben." Phil. Stud. 15, 1900,

p. 279.

E. C. CRITTENDEN and F. K. RICHTMYER.-"Color Photometry." Trans I. E. S. 11, 1916, p. 331.

GEORGE CLAUDE.-"Neon Tube Lighting." Trans. I. E. S., 8, 1913, p. 371. G. W. CASSIDY.-"Art and Science in Home Lighting." Trans. I. E. S., 10, 1915, p. 55.

JEAN ESCARD. "Mercury arc Modified to Give White Light." La Lum. Elec. 15, 1911, p. 236.