Stmmance-Abady “Flicker”? Photometer. 343 
adjustment. The colours then blend by persistence of the 
impression, but should the intensities again change, this 
retentive sense is dulled by the anxiety of the nerves controiling 
the pupil orifice to perform their work, an endeavour, 
however, frustrated by the rapid changing. It appears 
certain that an abnormal excitation of the nerves controlling 
the diaphragm has the effect of lessening the susceptibility 
of the retina to retention of impressions, whether this exci- 
tation is caused by glancing at a bright light or by gazing 
too long at the changing disk of the photometer. When the 
nerves are over-strained in this way the “flicker” does not 
disappear at all in many cases, although the point of equality 
of intensity is always unmistakable. Thus in using the pho- 
tometer it is suggested that the instrument shall be moved 
along the bar rather rapidly, the while observing the disk 
through the sighting-glass. The “motionless” point will 
then be apparent at once, and by a little practice (which in 
this, like everything else, generates confidence) an operator 
will swing the disk-box into its balance of intensity without 
an effort and without giving the sensitiveness of his retina 
time to suffer from the numbing effect of the flickerings or 
throbbing. These conditions point to the possibility of the 
eyes becoming under other circumstances of stress unable to 
appreciate the disappearance of the flicker. Obviously, if 
fatigue can produce such effects as described, certain con- 
ditions of health will act similarly. It may be that a bad 
liver will produce a constant flicker effect ; it is quite certain, 
however, that no circumstances short of partial or absolute 
blindness can prevent the point of equality of intensity being 
appreciated, although it may evidence itself in different 
appearances of the disk according to the varying conditions 
of the retina as mentioned above. 
The writer would submit, with all deference to the ex- 
pressed opinions of many of the world’s greatest scientists, 
that the Purkinje phenomenon does not affect the accurate 
working of the form of Flicker photometer shown, if indeed 
it affects any form of altcrnating photometer. This Purkinje 
effect may be briefly described in this way :—A right-angled 
wedge receives upon one face the reddish light from a pen- 
tane standard, and on the other the bluish light from an arc- 
lamp—we then adjust the relative positions of the arc-lamp, 
the pentane standard, and the wedge until we obtain what 
we consider is an equal illumination on the two adjacent 
sides of the wedge. If we then move in both these lights to 
half their distance from the wedge, we find that the retinal 
stimulation or apparent brightness of the two surfaces is no 
