eet”) 
IIl.—On the Gradual Production of Luminous Impressions on the Eye: Part I. 
being a description of an Instrument for producing isolated luminous impres- 
sions on the eye of extremely short duration, and for measuring their intensity. 
By Wittiam Sway, F.R.S.E., Professor of Natural Philosophy in the Uni- 
versity of St Andrews. 
(Read 4th April 1859.) 
In 1849 I presented to the Royal Society of Edinburgh a paper ‘“‘On the Gra- 
dual Production of Luminous Impressions on the Eye.” The object of that com- 
munication was to investigate the laws of the production of visual impressions. 
The subject was then new; for although the fact that light requires a certain 
time to produce its full effect on the eye had been noticed at comparatively early 
periods, yet no one, so far as | am aware, had attempted to measure that time ; 
and the whole subject of the production of visual impressions, regarded as a 
branch of experimental science, was quite untouched.* I have frequently wished 
to resume a subject of inquiry which seems to me to merit more attention than 
it has hitherto received, and which I have not as yet been able to discuss so com- 
pletely as I could have desired. Various portions of it demand more extended 
experiments. Among these may be specified, the examination of isolated im- 
pressions of shorter duration than my limited instrumental means had enabled 
me to observe; a more careful determination of the time required for light to 
produce its complete effect on the eye; and a series of observations on the eyes of 
various individuals, so that any personal peculiarity of vision, which might affect 
results derived exclusively from experiments on my own eye, might be eliminated. 
These, and other subjects of inquiry, I hope sooner or later to overtake ; and, in- 
deed, I had anticipated before the end of the present session of the Society to have 
been able to obtain some experimental results. In this I have been disappointed, 
owing to the instruments required for my observations having taken a much 
longer time to make than I had expected. 
The observations of impressions of very short duration is attended with much 
difficulty. The method of observation which I devised in 1848 is to cause a disc, 
with a sector cut out of it, to revolve with an uniform velocity between the eye 
and a luminous object. At each revolution a flash of light is perceived as the 
sector crosses the line of vision,—the time during which each flash acts on the 
eye depending on the angle of the sector and the velocity of rotation of the disc. 
The brightness of the resulting visual impression is ascertained by photome- 
* See Moigno, “ Repertoire d’Optique,” vol, ii., p. 563. 
VOL. XXII. PART I, I 
