8 REPORT—1845, 
explanation referred solely to very faint bands seen when the spectrum was out of 
focus, I had seen the bands perfectly distinct, and most vivid and intensely black, 
when the spectrum was in focus. The explanation, therefore, given in this memoir 
had nothing to do with the bands which I had discovered and described. Prof. Airy 
was accordingly led to resume the investigation; and he has published the results 
of it in a Supplement to his first paper, which appeared in the Philosophical Trans- 
actions for 1840. The following is the account which he gives of the results which 
he obtained :— ; 
“In the Second Part,” says Prof. Airy, ‘‘ of the Transactions for 1840, the Royal 
Society has published a memoir by me, explaining on the undulatory theory of 
light the apparent new polarity discovered by Sir D. Brewster, which explanation 
is based on the assumption that the spectrum is viewed out of focus; an assumption 
which corresponded to the circumstances of my own observations, and to those of some 
other persons. Since the publication of that memoir, I have been assured by Sir D. 
Brewster that the phenomenon was most certainly observed with great distinctness, 
when the spectrum was viewed so accurately in focus that many of Fraunhofer’s 
finer lines could be seen. This observation appeared to be contradictory to those of 
Mr. Talbot, cited by me in p. 226 of my memoir, as well astomy own. With the view 
of removing the obscurity that still appeared to embarrass this subject, I have con- 
tinued the theoretical investigations for that case which was omitted in the former 
memoir, namely, when the spectrum is viewed in focus, or when a=0; and I have 
arrived at a result which appears completely to reconcile the seemingly conflicting 
statements.””—Phil. Trans. 1841, p. 1. : 
Now, in the investigations which this paper contains, and which Prof. Airy con- 
siders satisfactory, there are two points which require special attention. The first of 
these is the assumption, necessary for the explanation, that even when any single 
point of the spectrum is seen accurately in focus, it forms a diffused image on the 
retina, the extent of the diffusion being exceedingly less than the interval between the 
bands. The supposition appears to me quite untenable, and one which cannot for 
a moment be admitted. The second point relates to the expression of a which 
Prof. Airy obtains for the interval between the bands; from which it follows, that 
this interval is inversely as the radius of the pupil, or the area of the object-glass. 
But the intervals have no such relation, and Prof. Airy does not say that such a re- 
lation was ever noticed in any of his experiments. I have made the experiment re- 
peatedly and carefully, and can state with confidence that the fringes do not vary 
with the diameter of the pupil or the apertures of the object-glass. Their interval re- 
mains the same, whether we look through a pin-hole or with the pupil in its fullest 
expansion ; and it is equally invariable when the aperture of the object-glass is made 
to vary from a quarter of an inch to four inches. Hence it follows that the system 
of bands to which Mr. Airy’s theory is applicable has no existence in nature; that 
the phenomena which I discovered are still unexplained by the undulatory theory, 
and may still be regarded as indicative of a new species of polarity, till they are 
brought under the dominion of some general principle. Since the publication of the 
two memoirs of Prof. Airy, I have devoted much time to the examination and mea- 
surement of the bands under consideration, and I have been led to the observation 
of many new and complex phenomena. [Sir D. Brewster read an account of these 
new phenomena.] I am still, however, as ignorant as ever of the cause of the sin- 
gular property to which this notice relates, though I have succeeded in tracing the 
phenomena to the true class of interferences to which they belong. 
Notice of Two New Properties of the Retina. 
By Siy Davip Brewster, F.R.S.L. §& E., Hon. M.RLA. 
One of these properties related to the inferior sensibility of the retina at that part 
of it which corresponds to the Foramen centrale of Soemmering, and which opens 
itself only when the eyes are directed to a faintly illuminated surface. The other 
property of the retina appeared after the observer’s eye had been impressed with the 
luminous stripes seen by looking out of a railway carriage in rapid motion at the 
stones, or other white bodies lying near the rails. When the eye is quickly shut 
