48 REPORT—1848. 
carriage, and quickly transfer the eye to the same lines further back, the stones or 
gravel or other objects are, for an instant, distinctly seen, just as we see distinctly 
rapidly revolving objects in the dark when they are, for an instant, illuminated by 
an electric flash, or seen in daylight through rapidly revolving slits. I have observed 
the same phenomenon less perfectly when travelling in a mail-coach, when its ve- 
locity was ten or twelve miles an hour. It may be also seen and studied by means 
of the revolving disc of the phenakistoscope. If we suddenly transfer the eye from 
the marginal parts of the disc, where the velocity is greatest, to the parts nearer the 
centre of rotation, where the velocity is less, we shall, for an instant, perceive di- 
stinctly the figures drawn upon that part of the disc. I have not been able to find 
a satisfactory explanation of this phenomenon. It may be connected with the 
transverse motion which appears upon closing the eyes when under these moving im- 
pressions*, or from an opposite secondary motion accompanying the primary one, 
the velocity of the primary one being diminished during the transference of the eye 
to lines moving with the velocity to which the impression is then reduced. 
The principal object of this notice is to describe a new fact which presented 
itself to me lately. If we look directly through a slit at the lines of moving stones, 
and suddenly look away from the slit, so as to see the moving lines through the slit 
by oblique or indirect vision, the stones will be distinctly seen. 
This neutral line, or line of compensation, arises from a quite different cause from 
the former, and admits of a satisfactory explanation. When the eye is turned away 
from the slit, a part of the retina, not previously subjected to any impressions, must 
see the stones for an instant, but only for an instant, as their motion immediately 
obliterates the first distinct impression. The neutral line thus produced is not so 
easily observed as in the other experiment, where the stones are seen by the part of 
the retina on which vision is most distinct, the vision being in the one case oblique 
and in the other direct. 
On the Vision of Distance as given by Colour. 
By Sir Davin Brewster, A.A, D.C.L., F.RS., §& V.PR.S. Edin. 
When the boundary lines on a map are marked with two lines of different colours, 
the one rises above or is depressed beluw the other, and the two lines appear to be 
placed at different distances from the eye. This remarkable effect is most clearly seen 
when we look with both eyes through a large reading-glass, spectacles being used along 
with it by those who require them. The more the two colours differ in refrangibility, 
the greater, and consequently the more distinctly seen, is the difference of distance 
at which the lines appear to be placed. The effect is finely seen in the coloured 
patterns of red and blue paper which Prof. Wheatstone has had executed on paper 
for exhibiting the mobility or shaking of one part of the pattern. The difference of 
distance of the coloured lines or spaces may be appreciated even with one eye. 
The explanation of this phenomenon is very simple. In binocular vision the con- 
vergency of the optic axis to different-points at different distances corresponding to 
the different points in the eye, to which the differently coloured rays are refracted, 
gives us the vision of a different distance for each coloured Jine, in the same manner 
as it is given in the stereoscopet. In monocular vision the distance is given by an 
analogous process to that by which the single eye sees distances. 
On the Visual Impressions upon the Foramen Centrale of the Retina. 
By Sir Davip Brewster, K.H., D.C.L., F.RS., & V.PRS. Edin. 
The foramen centrale of the retina is an opening in that membrane varying from the _ 
30th to the 50th ofan inch in diameter. Although there is no nervous membrane over 
this opening, it is nevertheless the part of the eye which gives most distinct vision, 
and hence it has been supposed that the retina is not the sole agent in conveying 
visual impressions to the sensorium. © Various attempts have been made to discover 
the existence of the foramen centrale by optical means, or to discover any effect pro- 
duced by it on the incident light. In making some experiments on vision I was led 
* See Report of 1845, Trans. of Sect. p. 8. 5 
+ See Edin. Transactions, vol. xv. p. 360. 
