PROCEEDINGS 
OF THE 
ROYAL SOCIETY OF EDINBURGH. 
vol. x. 1878-79. No. 104. 
Ninety-Sixth Session. 
Monday , 1 6th December 1878. 
Professor KELLAND, President, in the Chair. 
The following Communications were read 
I. On the Action of Light on the Iris. By William Ackroyd, 
F.I.C., &c. Communicated by Professor M‘Kendrick. 
Sect. I. It is well known that the movements of the iris are due 
to the stimulus of light, hut I am not aware that any experiments 
have been hitherto made to determine the approximate quantity of 
that agent necessary to bring about this involuntary action. The 
usual way of observation precludes refined experimenting, it being 
customary to watch the iris of another person or animal whilst under 
the influence of varying amounts of light, or one’s own iris by means 
of a mirror. Three methods will he described here, and I believe 
that one at least may afford a means of getting new data on this and 
other points. 
Sect. II. The first and second methods depend upon the following 
facts : — That, if a divergent bundle of rays emanate from a small 
surface or hole, Very near to the eye (say about 30 mm. off), this 
surface or hole is the apex of a cone of light whose base is the pupil; 
that every movement of the iris affects the area of this base, which 
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