DEVELOPMENTS IN ILLUMINATION—MILLAR,. 621 
the transactions of that society. The subject of nomenclature is 
especially vexing, and the art is fortunate in having the services of 
so distinguished a committee to assist in the adoption of sound 
definitions, symbols, and nomenclature. Pressure is being exerted 
with a view to the adoption of the metric system and some little 
progress appears to have been made toward this end. 
The principles of physical optics and of magnetic flux underlie 
many calculations * made in illuminating practices. Marked impetus 
was given to calculations of illumination by the application of the 
idea of luminous flux in commercial illumination design. In recent 
years the mathematics of the subject has been set forth repeatedly, 
and it may be said that calculations involved in illuminating engineer- 
ing work are perhaps further along toward complete development 
than is any other branch of the subject. 
The subject of costs” is a fundamentally important feature of the 
science of illumination, and questions of first cost and operating cost, 
including maintenance and depreciation, must have ‘the careful 
attention of the practitioner. The literature of this subject is rather 
meager, because of the difficulty of generalizing due to the marked 
influence which local conditions often exercise upon costs and due 
to the invidious form which cost discussions are likely to take. 
So much for the purely engineering aspects of the illuminatirg art. 
The engineering features are important, indeed essential, but other 
aspects are equally so. The subject of vision in all its ramifications 
forms an integral part of the science of illumination, a fact which is 
being given due recognition. Light must be correct in respect to 
intensity, direction, diffusion, color, and steadiness; and to the study 
of these qualities a knowledge of visual processes and methods of per- 
ception is essential.* Shade perception and visual acuity together 
with color perception have been studied and discussed to an extent 
which begins to make known some of the more important facts 
pertaining to vision. 
In this connection also the subject of contrast may be considered. 
A knowledge of the behavior of the human eye under various condi- 
tions of contrast is all essential to the science of illumination. There- 
fore the study of reflection and absorption of ight and of brightness 
1 Calculations: “A rectilinear graphical construction of the spherical reduction factor of a lamp;” Ken- 
nelly; Trans. Ulg. Eng. Soc., 1908, p. 243. ‘The calculation of illumination by the flux of light method;” 
Cravath and Lansingh; Trans. Ilg. Eng. Soc.,1908,p.518. “Calculating and comparing lights from various 
sources;” Hering; Trans. Ilg. Eng. Soc., 1908, p. 645. ‘The law of conservation as applied to illumination 
calculations;” McAllister; Trans. Illg. Eng. Soc., 1911, p. 703. ‘ 
2 Costs: “The analysis of performance and cost data in illuminating engineering; Harrison and Magd- 
sich; Trans. Illg. Eng. Soc., 1911, p. 814. 
* Visual processes: “ Iffects of light upon the eye;” Seabrook; Trans. Illg. Eng. Soc., 1908, p.157. “Eye- 
strain and artificial ilumination;” Krawell; Trans. Illg. Eng. Soc., 1908, p. 212. “Artificial illumination 
from a physiological point of view;” Standish; Trans. Illg. Eng. Soc., 1908, p. 254. ‘“Eyestrain;” Pyle; 
Trans. Ulg. Eng. Soc., 1909, p. 447. “Physiological effects of radiation;” Steinmetz; Trans. Illg. Eng. 
Soc., 1909, p. 683, “The psychology of light;”” Woodworth; Trans. Illg. Eng. Soc., 1911, p. 437. 
