342 Messrs. Simmance and Abady on the 
“‘ flicker”’ photometer, the operator was in some way testing 
acuteness of sight, and not relative intensities. Acuteness of 
sight naturally does come into play in this, as in all other 
matters in which sight is used at all, but when we remember 
that with lights of the same tint we apparently see in the 
photometer a white surface which is alternately illuminated 
by the two lights, we must conclude that when this alterna- 
tion is invisible (that j is to say, when we can see no change) 
the reason is because both lights are of the same power. 
Naturally, abnormally acute vision would perceive more 
minute changes of intensity, but this is a factor always present 
in any investigation of the sort. 
It appears quite certain that the relative intensities of two 
lights, whether of the same colour or different, can be 
accurately and easily gauged by the method. An interesting 
experiment has recently been conducted by Mr. Roxburgh 
and Mr. Young, of the Ophthalmic Hospital. Readings 
upon this photometer were made independently by, firstly, 
Mr. Jacques Abady, who is an experienced photometrist, 
but suffering from astigmatism ; secondly, a gentleman 
with a pupil dilated by treatment with atropin ; thirdly, a 
patient from the hospital, absolutely colour-blind ; and, 
fourthly, by the writer, whose sight is considerably impaired, 
but who has some experience with photometer reading. All 
the results coincided, and this not only when the lights were 
of the same white tint, but with red against white, green 
against white, and green against red. A test of very dark 
blue against red showed slight discrepancies, owing, probably, 
to the great obscuration of the light by the blue, almost 
black, glass, but even these were no more in importance than 
would probably have occurred when using a Lummer-Brodhun 
or Bunsen disk with lights of the same colour. 
The method of reading with this photometer resolves itself 
into the simple question as to whether the disk of light seen 
through the sighting tube is moving or is motionless. An 
observer who makes a reading for the first time, when he 
fully understands that he is required to do no more than this, 
can read with as great facility and accuracy as an experienced 
operator, and it is this facility which gives value to an appa- 
ratus intended for testing the high intensity lights in vogue 
at the present time, and also colour effects. 
As to the causes of the phenomenon, they, no doubt, are 
simple on the surface, but may be more complex when closely 
considered. ‘The pupil of the eye cannot adjust itself to the 
rapidly changing intensities, and when the balance is struck, 
the light, being unchanging and motionless, permits of pupil 
