British Association for the Advancement of Science. 107 



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 

 his invention. 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 bees-wax, which rendered 

 the apparatus air-tight. 



On a new case of interference of light, by Prof. Powell. The 

 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 hands ; 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. P. slips a zinc tube, which passes through a brass 

 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 



