Photometry of Lights of Different Colours. 357 



the ordinates are critical frequencies in cycles per second. 

 The three circles B, 0, and D show the illuminations at 

 which the three critical frequency luminosity curves were 

 obtained. The reversed Purkinje effect occurs in the upper 

 or " cone " region, the Purkinje effect in the lower or 

 " rod " region. In Hay craft's work the ends of the spectrum 

 were always in the " rod " region, so that the reversed 

 Purkinje effect would show itself only as a warping of the 

 medium illumination parts of the curves by an amount 

 easily to be mistaken for errors of measurement. 



Before leaving consideration of fig. 3 attention is called 

 to curve N. This was made without the artificial pupil of 

 1 sq. mm. aperture. It furnishes a means of evaluating the 

 " illumination units " here used, in terms of metre candles 

 illumination of the standard white magnesium oxide surface. 

 To reduce illumination units to metre candles it is necessary 

 to divide by from 7 to 8 over the range of these experiments. 

 This figure of course applies to the writer's eye, and will be 

 different for other eyes in proportion to the relative natural 

 pupillary apertures of different observers. Since in working 

 with critical frequencies the field is illuminated for only one 

 half the time, the flicker and equality of brightness illumi- 

 nation units should be doubled in order to use this diagram. 



While the discovery of the relation of the Purkinje effect 

 and its opposite to the two regions previously found by 

 Porter is a distinct step, further insight into the phenomena 

 follows from more detailed study. On examination of this 

 data it is obvious that the facts presented point to different 

 critical frequency-illumination relations for different colours. 

 Moreover, the relation in the rod region must be different 

 from that in the cone region, the change taking place at 

 about 2-5 I. U. 



To test this hypothesis a set of critical frequency 

 determinations was made for various spectral colours. 

 Illuminations were reduced by neutral tint screens * placed 

 over the spectroscope slit. The results shown in fig. 4 bear 

 out the surmise. In the " cone " region the different 

 colours give straight lines (to within the errors of measure- 

 ment) which are differently inclined in such manner as to 

 give the reversed Purkinje effect. At critical frequencies 

 of from 18-12 cycles per second these lines change their 

 directions. Their new relative inclinations are such as 

 to give the true Purkinje effect. The most remarkable 



* " A Form of Neutral Tint Screen," Herbert E. Ives & M. Luckiesh, 

 Phys. Review, May 1911 . 



Phil Mag. S. 6. Vol. 24. No. 141. Sept, 1912. 2 B 



