British Association for the Advancement of Science. = 
Prof. Powell presented a report on refractive indices, containing 
the mean results of many series of observations for a considerable 
range of substances. The author made a few preliminary obser- — 
vations on the nature of the inquiry, especially with reference to 
some points of dispute respecting the identification of certain rays” 
of the spectrum, which had been discussed by Sir D. Brewster 
and himself. 
Mr. Nasmyth exhibited a Plate-glass pneumatic speculum, of 
_ hisinvention. The glass was three feet three inches in diameter, 
and three sixteenths of an inch thick. It was placed on a con- 
cave cast-iron bed and fastened in with srs which rendered 
_ the apparatus air-tight. ‘ 
es On a new case of interference e light, by Prof. Powell. The na 
~ author observed that when a prism of one substance was opposed ~ 
to another, slightly differing in dispersive power, (as plate glass 
and oil of sassafras,) so as to produce a partial achromatism, in _ 
the colored edges, which appeared on either side of the white 
image of a narrow line of light, when viewed through a small 
telescope, there were formed dark bands ; about four or five being 
Visible on each edge, parallel to the line of light. The explana- 
tion is easy, when we consider that of the parallel pencil of each — 
primary ray which enters the eye, (in breadth equal to the aper 
ture of the pupil,) the rays which have traversed greater thick-- 
nesses of the first prism, traverse less thicknesses of the second ; 
and thus have their retardations so nearly compensated, as to be 
in a condition to interfere and produce the dark bands observed. 
In the same manner Prof. P. explained the analogous optical phe- — 
nomena observed by Sir D. Brewster, and stated at previous meet- 
ings of the Association. 
Dr. Andrew Ure offered some account of his mode of measur- 
ing, by means of photogenic paper, diffuse daylight compara-— 
tively at any time and place. He also recounted a series of ex~ 
periments which he had made to determine the fluency or viscid- 
ity of different liquids at the same temperature, and of the same 
liquids at different temperatures. 
Mr. W. J. Frodsham exhibited and described his improved com- 
pound pendulum. It is an ordinary pendulum, with a steel rod, _ 
over which Mr. F. slips a zine tube, which passes through a aks 
bob, and rests on the adjusting screw at the lower end of the rod, 
the bob being fastened at the centre by two connecting rods of ~ 
