MEASUREMENTS OF LIGHT.

Ashburton Guardian, Volume XXiX, Issue 7727, 22 February 1909, Page 1

MEASUREMENTS OF LIGHT.

Photometry, or measurement of tlio intensity of light, is becoming increasingly important. Once hardly; more than a laboratory experiment, | it is now part of regular industrial :procedure in the. manufacture and comparison of various types of electric lamp. In comparing the lamp with the standard unit to ascertain its ''candle pow*r" the human eye is obliged in all practical systems hitherto dev^ed. This is objectionable, for the eye jectto all sorts of faults; but attempts to substitute some form of miaaurement depended on chemical action, vaiiation of electrical resistance,: etc., have been failures These nisthocU. measure, not the light, that is, the part of the total radiant energy appreciable by the eye, but some other parts of it with different limits, dependent on the method used. A French, physi- | cian, C. Fery, has been attempting with somo success to cut off alhradiation not appreciable by the eye by using absorbent liquors and then to measure the remainder in one of the various chemical or electrical ways already proposed. A solution of acetate per of proper concentration has been round to serve very well, and measurements of the transmitted radiation made with Boy's radiopmicrpmeter agree almost perfectly with those made with tho human eye. The radio-nn-crometer is an adaptation, for purposes of delicate measurement of the Grookes radiometer—tho revolving mica: vanea in an exhausted bulb so often displayed in the windows of opticians.