600 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 
sion. If this proportionality between the duration and the apparent brightness of a 
light be supposed to extend beyond the limits of the experiments, so as to include 
nearly the whole time required for the production of a complete impression, it 
would obviously follow that light requires about th of a second to produce its 
full effect on the eye; and this conclusion, it will be observed, agrees with the 
result of direct observation. 
The following inferences may be derived from the laws of vision which 
have now been investigated. 
1. Personal Equation in Astronomy. 
It is well known, that different observers assign different times to the occur- 
rence of the same astronomical phenomenon ; as, for example, the passage of a 
star across the meridian wire of a transit instrument. The correction to be ap- 
plied to reduce the observations, or personal equation, as it is termed, frequently 
amounts to a considerable quantity. 
It might at first be supposed, that this discrepancy between the results of 
different observers, may be occasioned by light acting on their eyes with unequal 
degrees of rapidity. But on considering the manner of observing the transit of a 
star, it will appear that this explanation is insufficient. In order to estimate the 
exact time at which the star passes one of the wires, the observer endeavours to 
recollect the position of the star on one side of the wire, at the instant when he 
heard the clock beat. At the next beat of the clock, the star has passed to the 
other side of the wire; and the observer then, by the eye, subdivides into equal 
parts the space between the positions occupied by the star at the successive beats 
of the clock, and estimates how many of those parts are contained in the interval 
between the wire and the first position of the star. The magnitude of that interval 
estimated in this manner, determines the fraction of a second to be added to the 
time given by the clock. If, then, the discrepancy between the results of different 
observers is to be regarded as a phenomenon of vision, it must depend upon some 
cause which displaces the image of the star, and thereby alters its apparent dis- 
tance from the wire. Now as the star passes across the field of the telescope, its 
light falls successively upon different parts of the retina, illuminating each portion 
for a very small space of time ; and if light acted on the eye of one observer more 
rapidly than on that of another, the obvious consequence would be that the 
image of the star would appear brighter to the person whose retina was most 
quickly impressed by light. The only other effect which the gradual action of 
light on the eye seems capable of producing, is to render the advancing edge 
of the image of the star so faint, owing to the extremely short time during 
which its light acts on the eye, as to become imperceptible when contrasted with 
the succeeding parts of the image: for these fall upon points of the retina over 

