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rate of flow of electric current and have a practical conception of the magnitude of the unit. In dealing with illumination measurements it is more practical to have a conception of the quantity of light represented by one lumen, than it is to be able to tell precisely what a lumen is. Obviously, it is of advantage to standardize certain units so that relations of magnitude can be expressed and understood with precision, although the value we arbitrarily assign as a standard is of little importance except from this standpoint.
The Candle.
A generation or two ago, when new light sources began to supersede the candle, it was most natural that the illuminating power of these new sources should be expressed in terms of the candle familiar to all. It is probable that the very first comparisons of two light sources were made by setting up the two lamps in the line of vision and gauging them by means of the eye, the most natural direction in which to look at the sources being the horizontal.
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Fig. I
Only a slender cone of light reaches the eye
A glance at Fig. i shows that the eye (an extremely fallible instrument of light measurement at its best) is capable of measuring only a very slender cone of light at one time; in fact, if the eye is an appreciable distance from the source, the cone i?6'T' becomes virtually a single line.
While there are an infinite number of directions from which the eye might look at the source, the light-giving power in a horizontal direction was made the basis of comparisons, and the strength of the light in this direction from a candle made according to certain definite specifications, was arbitrarily chosen as the unit of intensity and called a candle. The newer illuminants appearing on the scene were rated according to their strength in this same direction and were stated to give so many candles. This rating of an illuminant is made by means of an instrument known as a photometer, a description of which will follow later. One essential point to remember in this connection is that the candle-power of a lighting unit represents the intensity in one direction only. In practice it has been customary for years to rotate the illuminant about a vertical axis while the candle-power was being determined and the result was known as the mean or average horizontal candle-power but even this determination gives an average value of the intensity in the horizontal direction only.
To carry our conception of candle-power a little further, let us assume three conditions.
In the first case (Fig. 2A) we have on the left a standard candle and on the right a photometer pointed toward the candle. From what has
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