1901.] Polish. 663 



WEEKLY EVENING MEETING, 

 Friday, March 29, 1901. 



The Eight Hon. Sir James Stirling, M.A. LL.D., 

 Vice-President, in the Chair. 



The Right Hon. Lord Rayleigh, M.A. D.C.L. LL.D. F.R.S. M.B.I. 



PROFESSOR OF NATURAL PHILOSOPHY E.I. 



Polish. 



The lecture commenced with a description of a home-made spectro- 

 scope of considerable power. The lens, a plano-convex of 6 inches 

 aperture and 22 feet focus, received the rays from the slit, and finally 

 returned them to a pure spectrum formed in the neighbourhood. The 

 body of the prism was of lead ; the faces, inclined at 70°, were of 

 thick plate-glass cemented with glue and treacle. It was charged 

 with bisulphide of carbon, of which the free surface (of small area) 

 was raised above the operative part of the fluid. The prism was 

 traversed twice, and the effective thickness was 5^ inches, so that 

 the resolving power corresponded to 11 inches, or 28 cm., of CS 2 . 

 The liquid was stirred by a perforated triangular plate, nearly fitting 

 the prism, which could be actuated by means of a thread within 

 reach of the observer. The reflector was aflat, chemically silvered 

 in front. 



So far as eye observations were concerned, the performance was 

 satisfactory, falling but little short of theoretical perfection. The 

 stirrer needed to be in almost constant operation, the definition usually 

 beginning to fail within about 20 seconds after stopping the stirrer. 

 But although the stirrer was quite successful in maintaining uni- 

 formity of temperature as regards space, i.e. throughout the dispersing 

 fluid, the temperature was usually somewhat rapidly variable with 

 time, so that photographs, requiring more than a few seconds of 

 exposure, showed inferiority. In this respect a grating is more 

 manageable. 



The lens and the faces of the prism were ground and polished 

 (in 1893) upon a machine kindly presented by Dr. Common. The 

 fiat surfaces were tested with a spherometer, in which a movement of 

 the central screw through xwinro" i ncn could usually be detected by 

 the touch. The external surfaces of the prism faces were the only 

 ones requiring accurate flatness. In polishing, the operation was not 

 carried as far as would be expected of a professional optician. A 



2 p 2 



