Photometry of Lights of Different Colours. 163 



length of *574/*); the divided head (4) is turned to give a slit- 

 opening of 10 units (01 mm.) : the lamp (10) is then moved 

 until the two halves of the field are, as nearly as can be 

 estimated, equally bright, at which point the lamp is fixed. 

 A set of readings is then made, first by equality of brightness, 

 then, with the disk rotating, by the flicker method, intensities 

 being matched by changing the width of the slit (3). The 

 wave-length drum is then changed to a point in the red, 

 next to a point in the green or blue; and so alternating from 

 one side of the original point to the other, similar double sets 

 are made. When the whole spectrum as far as practicable 

 is covered, a set of slit-widths is obtained, the reciprocals of 

 which are as the brightnesses of the colours. By this means 

 the illumination is constant throughout any one measurement 

 — an important condition, which has been overlooked by 

 several investigators. 



Three sizes of field are used : one corresponding to the 

 whole prism-face, an oblong 18 X 24 mm., somewhat larger, 

 as viewed at 20 cm., than the angle subtended by the macula 

 lutea; the second, a circular opening of 16 mm. diameter, 

 subtending at a distance of 20 cm. an angle a little less than 

 the macula lutea ; the third, a circular opening of 6*5 mm. 

 diameter, corresponding to the angular size of the fovea. At 

 each illumination a set of equality of brightness and flicker 

 measurements is made for each field-size. The making of 

 such a set for one field-size consumes a morning or afternoon, 

 and but one can be made safely in a day without unduly 

 fatiguing the observer. 



The measurements so obtained are subject to several 

 corrections. These are : first, that for slit-width ; second, 

 that for prismatic dispersion ; third, for the selective absorption 

 of the prism ; fourth, for the energy distribution of the source 

 whose spectrum is used. The illumination as measured by the 

 intensity and distance of the lamp (8) must be corrected for 

 the loss by absorption through the telescope lens. 



To determine the slit-width correction it is necessary to 

 know the approximate luminosity curve and the portion of 

 the spectrum included in each measurement. The latter was 

 determined for the whole range of slit-widths used, and for 

 the largest slit was found to amount to *03/a at '66 fi, and to 

 •01 yLt at '49 fi. In the yellow, where the luminosity curve 

 changes its direction most rapidly, the greatest width used is 

 '016 /j,. In this region the correction calculated from the 

 approximate luminosity curves is 0*6 of one per cent. This, 

 the largest correction, is of the order of magnitude of the 

 errors of observation, and may therefore be neglected. 



M2 



