90 ’ REPORT—1850. 
rings when formed between a speculum and a glass plate. Mr. Airy’s paper is pub- 
lished in the Cambridge Philosophical Transactions. M. Jamin has since been led 
to the same result, apparently by a method similar in principle to that of Mr. Airy. 
In repeating Mr. Airy’s experiment, the author experienced considerable difficulty in 
observing the phenomenon. The object of the present communication was to point 
out an extremely easy mode of deciding the question experimentally. Light polar- 
ized at an azimuth of about 45° to the plane of reflexion at the surface of the metal 
was transmitted, after reflexion, through a plate of Iceland spar, cut perpendicular 
to the axis, and analysed by a Nicol’s prism. When the angle of incidence was the 
smallest with which the observation was practicable, on turning the Nicol’s prism 
properly the dark cross was formed almost perfectly ; but on increasing the angle of 
incidence it passed into a pair of hyperbolic brushes. This modification of the rings is 
very well known, having been described and figured by Sir D. Brewster in the Phi- 
losophical Transactions for 1830. Now the question at issue may be immediately 
decided by observing in which pair of opposite quadrants it is that the brushes are 
formed, an observation which does not present the slightest difficulty. In thisway — 
the author was led to Mr. Airy’s result, namely that as the angle of incidence in- . 
creases from zero, the phase of vibration of light polarized in the plane of incidence 
is accelerated relatively to that of light polarized in a plane perpendicular to the 
plane of incidence. ----.-... 
On a Fictitious Displacement of Fringes of Interference. 
By Professor Stoxes, M.A. 
The author remarked that the mode of determining the refractive index of a plate 
by means of the displacement of a system of interference fringes, is subject to a 
theoretical error depending upon the dispersive power of the plate. It is an extremely 
simple consequence (as the author showed) of the circumstance that the bands are 
broader for the less refrangible colours, that the point of symmetry, or nearest ap- 
proach to symmetry, in the system of displaced fringes, is situated in advance of the 
position calculated in the ordinary way for rays of mean refrangibility. Since an 
observer has no other guide than the symmetry of the bands in fixing on the centre 
of the system, he would thus be led to attribute to the plate arefractive index which 
is slightly too great. 
The author has illustrated this subject by the following experiment. A set of 
fringes, produced in the ordinary way by a flat prism, were viewed through an eye- 
piece, and bisected by its cross wires. On viewing the whole through a prism of 
moderate angle, held in front of the eye-piece with its edge parallel to the fringes, an 
indistinct prismatic image of the wires was seen, together with a distinct set of 
fringes which lay quite at one side of the cross wires, the dispersion produced by the 
prism having thus occasioned an apparent displacement of the fringes in the direc- 
tion of the general deviation. 
In conclusion, the author suggested that it might have been the fictitious displace- 
ment due to the dispersion accompanying eccentrical refraction, which caused some 
philosophers to assert that the central band was black, whereas, according to theory, 
it ought to be white. A fictitious displacement of half an order, which might readily 
be produced by eccentrical refraction through the lens or eye-piece with which the 
fringes were viewed, would suffice to cause one of the two black bands of the first 
order to be the band with respect to which the system was symmetrical. 
On Haidinger’s Brushes. By Professor Stoxes, M.A. ᾿ 
It is now several years since these brushes were discovered, and they have βίποθ 
been observed by various philosophers, but the author has not met with any ob- — 
servations made with a view of investigating the action of different colours in — 
producing them. The author’s attention was first called to the subject, by ob- — 
serving that a green tourmaline, which polarized light very imperfectly, enabled him 
to sce the brushes very distinctly, while he was unable to make them out with ἃ 
brown tourmaline which transmitted a much smaller quantity of unpolarized light. — 
He then tried the effect of combining various coloured glasses with a Nicol’s prism. 
A red glass gave no trace of brushes. <A brownish yellow glass, which absorbed 
only a small quantity of light, rendered the brushes very indistinct. A green glass — 
enabled the author to see the brushes rather more distinctly than they were seen in 
