620 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
widely different color values. There is a great need for a series of 
such standards which shall be authoritative by reason of the auspices 
under which they have been derived as well as by official designation. 
A number of laboratories are engaged in the study of this problem 
of heterochromatic photometry, and- while concrete results in the 
establishment of such standards are not available yet, progress must 
be recorded in that the need for such standards is now definitely 
established and work is under way which should result ultimately 
in meeting this need. Present indications are that a range of cali- 
brated color screens offers a most practical solution of this problem. 
Standards of light ? may be classified as primary, representative, 
and working standards. Primary standards, or those reproducible 
from specifications, are at present flame standards, respectively 
candles, the Hefner lamp, and the Pentane lamp. There have been 
no important developments in the way of primary standards of light 
in recent years, although certain means of arriving at a superior 
primary standard have been suggested and some research work has 
been done with that end in view. It is generally recognized that 
none of the existing primary standards of light is entirely satisfactory 
and that there is need for the development of a new and superior 
standard. Representative standards have been adopted and the 
so-called international candle is the official unit of light in England, 
France, and the United States. It is the result of standardization 
work of the past few years, and the unit is now represented by groups 
of seasoned, calibrated incandescent electric lamps held at the 
official laboratories of these three countries. These form a reason- 
ably accurate and safe standard for light of one-color value. From 
them working standards are derived which accurately duplicate 
the value of the standard lamps and which are now available for 
general use of all who require them. 
A start toward adopting a reasonable system of units and nomen- 
clature ? was made at the Geneva Electrical Congress in 1896. The 
committee on nomenclature and standards of the Illuminating 
Engineering Society has been actively engaged in the furthering of 
this work. That considerable progress has been made will be testi- 
fied by the several annual reports of the committee to be found in 
1 Photometrical laboratories: ‘‘Photometrical laboratories of National Bureau of Standards;” Stratton 
and Rosa; Trans. A. I. E. E., 1905, p. 999. “A testing laboratory in practical operation;” Sharp; Trans. 
A.I.E.E.,1905, p.1051. ‘“Photometrical laboratory of the United Gas Improvement Co.;’’ Bond; Trans. 
Tilg. Eng. Soc., 1909, p. 619. “Physical laboratory of the National Electric Lamp Association;” Hyde; 
Trans. Illg. Eng. Soc., 1909, p. 631. 
2 Standards of light: ‘‘Standards oflight;’’? Nichols; International Electrical Congress, 1904; Steinmetz; 
Trans. A.I.E.E.,1908, p.1319. ‘Report of the committee on nomenclature and standards ;” Trans. Illg. 
Eng. Soc., 1909, p. 520. ‘Heterochromatic photometry and a primary standard of light;” Ives; Trans. 
Ilig. Eng. Soc., 1912, p. 376. 
3 Units and nomenclature: “Reports of the committee on nomenclature and standards ;” Trans. Ilg. 
Eng. Soc., to date. “The concepts and terminology of illuminating engineering;” Sharp; Trans. Illg. 
Eng. Soc., 1907, p. 414. 
