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<p>The author points out the ambiguity in the meaning attached to the word "light". It is sometimes applied to the subjective sensation, and sometimes to the objective stimulus causing, or capable of causing, this sensation. The relation, however, of light to radiation involves primarily the concept of the luminous equivalent of radiation which is capable of exact definition. But while the heat equivalent of mechanical energy is a fixed quantity, the luminous equivalent of radiation varies enormously with the wave-length of the radiation concerned and to a slight extent with its intensity as well. The principle of the conservation of energy does not apply to light either objective or subjective. Light is not a simple multiple of radiation, but a complicated function of
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<br>Subjects: photometry; rays and radiation; vision</div>
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