116 Lord Rayleigh [March 24, 



WEEKLY EVENING MEETING, 



Friday, March 24, 1899. 



Sib Frederick Bramwell, Bart., D.C.L. LL.D. F.R.S., 

 Honorary Secretary and Vice-President, in the Chair. 



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

 Professor of Natural Philosophy, B.I. 



Transparency and Opacity. 



One kind of opacity is due to absorption ; but the lecture dealt 

 rather with that deficiency of transparency which depends upon 

 irregular reflections and refractions. One of the best examples is 

 that met with in Christiansen's experiment. Powdered glass, all 

 from one piece and free from dirt, is placed in a bottle with parallel 

 flat sides. In this state it is quite opaque ; but if the interstices 

 between the fragments are filled up with a liquid mixture of bisulphide 

 of carbon and benzole, carefully adjusted so as to be of equal refrac- 

 tivity with the glass, the mass becomes optically homogeneous, and 

 therefore transparent. In consequence, however, of the different 

 dispersive powers of the two substances, the adjustment is good for 

 one part only of the spectrum, other parts being scattered in trans- 

 mission much as if no liquid were employed, though, of course, in a 

 less degree. Tbe consequence is that a small source of light, backed 

 preferably by a dark ground, is seen in its natural outlines but strongly 

 coloured. The colour depends upon the precise composition of the 

 liquid, and further varieswith the temperature, a few degrees of warmth 

 sufficing to cause a transition from red through yellow to green. 



The lecturer had long been aware that the light regularly trans- 

 mitted through a stratum from 15 to 20 mm. thick was of a high 

 degree of purity, but it was only recently that he found to his 

 astonishment, as the result of a more particular observation, that the 

 range of refrangibility included was but two and a half times that 

 embraced by the two D-lines. The poverty of general effect, when 

 the darkness of the background is not attended to, was thus explained ; 

 for the highly monochromatic and accordingly attenuated light from 

 the special source is then overlaid by diffused light of other colours. 



More precise determinations of the range of light transmitted 

 were subsequently effected with thinner strata of glass powder con- 

 tained in cells formed of parallel glass. The cell may be placed 

 between the prisms of the spectroscope and the object-glass of the 

 collimator. With the above mentioned liquids a stratum 5 mm. thick 

 transmitted, without appreciable disturbance, a range of the spectrum 

 measured by 11 '3 times the interval of the D's. In another cell of 



