PHOTOMETRY AND ELECTRIC LIGHTING. 309 



second across an area of one square centimeter at a distance of 

 one meter from the candle, it follows that only about 2 per cent, 

 of the energy radiated by a standard candle lies within the limits 

 of the visible spectrum, that is, only about 2 per cent, of the radi- 

 ation from a standard candle is light. 



The luminous intensity of a beam of light is presumably measured 

 by the intensity of the light sensation it can produce, but the in-" 

 tensity of the light sensation produced by a given beam of light 

 is extremely indefinite. A given beam of light entering the eye 

 may produce a strong or a weak sensation depending upon mani- 

 fold individual peculiarities of the person and upon the degree of 

 fatigue of the retina, and the vividness of the sensation depends 

 upon the extent to which it is enhanced by attention. Our sen- 

 sations are really not quantitative in the physical meaning of that 

 term ; in fact they enable us merely to distinguish objects, to judge 

 whether things are alike or unlike, and the certainty and precision 

 with which we can do this is exemplified in every outward aspect 

 of our daily life. The ratio of the luminous intensities of two beams 

 of light is measured by using a device to alter in a known ratio the 

 physical intensity of one beam until it gives, as nearly as one can 

 judge, a degree of illumination on a screen which is equal to (like] 

 the illumination produced by the other beam. 



126. Photometry. The measurement of the light emitted by 

 a lamp is called photometry. This measurement is always made 

 by comparing a beam of light from the given lamp with a beam 

 of light from a standard lamp, as explained in the previous para- 

 graph, and the physical device there referred to is called ^.photom- 

 eter. The comparison of the total light in a beam from a given 

 lamp with the total light in a beam from a standard lamp is called 

 simple photometry ; whereas the comparison wave-length by wave- 

 length throughout the spectrum is called spectro-photometry. A 

 fundamental difficulty* in simple photometry is that different 



* A method for overcoming this difficulty by means of what is called the flicker- 

 photometer is described in Nichols & Franklin's Elements of Physics, Vol. III., p. 

 128. 



