162 Mr. H. E. Ives on tie 



by monochromatic light of a purity determined by the 

 relative size of collimator and eye-slit (3 and 1). The 

 wave-length incident on the eye-slit is changed by motion 

 of the drum (5). The size of the collimator slit is changed 

 by turning the drum (4) which is provided with a divided 

 head. At (6) is a thin metal disk of the shape shown in 

 separate sketch. This has carefully bevelled edges and is 

 painted white. Periodically the disk is smoked over burning 

 magnesium wire, so that the surface used is of white mag- 

 nesium oxide. At (7) is a photometer track, carrying a 

 movable incandescent lamp (8). The disk is connected by a 

 belt with a direct-current series-wound motor in series with 

 a variable resistance, and the same belt passes over the 

 pulley of a Weston electric tachometer. At (9) is a piece 

 of finely ground glass; at (10) a frosted bulb tungsten lamp, 

 movable on a photometer track (11). At (12) may be placed 

 diaphragms of any desired size. 



For light source at (10) the requirements to be met were: 

 first, as high intensity as possible; second, as white a source 

 as possible, since prismatic dispersion and the selective 

 absorption of the prism glass both decrease the amount of 

 blue light for practicable slit-widths. A tungsten lamp 

 burning at normal voltage was ultimately decided upon. At 

 frequent intervals this was checked for colour upon a photo- 

 meter by ascertaining the voltage to give the integral colour 

 of three standards which were not burned at any other time. 

 This proved fairly satisfactory, although some change in 

 distribution undoubtedly occurred during the investigation, 

 due partly to blackening of the bulb. 



For light source at (8) carbon lamps were used, of uniform 

 colour, ranging in candle-power from 65 to 5. These placed 

 at various positions along the photometer bench permitted 

 variations of illumination from 1 to 600 metre candles. The 

 colour chosen was that of the incandescent lamp secondary 

 standards at present in use, namely that of a 4*85 watts 

 per mean spherical candle treated carbon, oval anchored 

 filament. Use of an unsaturated colour of* a particular hue 

 will of course give results applying only to comparisons with 

 such a standard. The complete study of the question will 

 probably demand investigation of any possible effects of 

 altering the character of the standard light. 



The method of obtaining the luminosity curves is as 

 follows : — The lamp (8) is placed to give a certain illumi- 

 nation ; the disk (6) is turned so that the photometric field 

 is bisected ; the drum (5) is set at a point near the middle of 

 the spectrum (taken always arbitrarily to give a mean wave- 



