TEANSACTIOMS OF SECTION A. 585 



lengths of various lines are too small hj more than one part in ten thousand — an 

 amount which is inconveniently large. Angstrom's map contains so few lines, and 

 difl'ers so widely in appearance from the spectrum as seen in a modern spectroscope, 

 that it is quite confusing ; and a reference to a line by its place on his map is very 

 insufficient means of identitication, besides giving a value for the wave-length 

 which is quite far from the truth. 



10. Twin-Prisms for Polarimeters. 

 By Professor SiLVANUs P. Thompson, D.Sc. 



The author described two new forms of twin-prism for use in saccharimeters 

 and polarimeters. The first, intended for use in a manner similar to Laurent's 

 half-shadow prism, consisted of two rectangular polarisers, cut on the plan de- 

 scribed by the author at the Association meetings in 1881 and 1886, placed side by 

 side, and having the planes of polarisation in their respective fields inclined at 90° 

 to each other. In the second twin-prism the angle between the planes of the two 

 prisms was about 2^° ; and it was intended to be used like the prisms of Jellett or of 

 Cornu. The method of constructing polarisers previously described by the author 

 had special advantages for this purpose, as the polarised field was more homogeneous 

 than that of the Nicol prism, and it was very easy to make the required adjust- 

 ments of angles. 



11. On the Existence of Reflection when the Helatlve Refractive Index is 

 Unity. By Lord Rayleigh, LL.D., Sec.R.S. 



The copious undisturbed transmission of light by glass powder when surrounded 

 l)y liquid of the same index, as in Christiansen's experiment, suggests the question 

 whether the reflection of any particular ray is really annihilated when the rela- 

 tive index is unity for that ray. Such would be the case according to Fresnel's 

 formulae, but these are known to be in some respects imperfect. Mechanical theory 

 would indicate that when there is dispersion, reflection would cease to be merely a 

 function of the index or ratio of wave-velocities. AVe may imagine a stretched 

 string vibrating transversely under the influence of tension, and in a subordinate 

 degree of stiflness, to be composed of two parts so related to one another in respect 

 of mass and stiflness that the wave-velocity is the same in both parts for a specified 

 wave-length. But, as it is easy to see, this adjustment will not secure the complete 

 transmission of a train of progressive waves incident upon the junction, even when 

 the wave-length is precisely that for which the velocity is the same. 



The experiments that I have tried have been upon plate glass immersed in a 

 mixture of bisulphide of carbon and benzole, of which the first is more refractive 

 and the second less refractive than the glass ; and it was found that the reflection 

 of a candle-flame from a carefully cleaned plate remained pretty strong at moderate 

 angles of incidence, in whatever proportions the liquids were mixed. 



For a closer examination the plate was roughened behind (to destroy the second 

 reflection), and was mounted in a bottle prism in such a manner that the incidence 

 <;ould be rendered grazing. When the adjustment of indices was for the yellow, 

 the appearances observed were as follows : if the incidence is pretty oblique, tlie 

 reflection is total for the violet and blue ; scanty, but not evanescent, for the yellow ; 

 more copious again in the red. As the incidence becomes more and more nearly 

 grazing, the region of total reflection advances from the blue end closer and closer 

 upon the ray of equal index, and ultimately there is a very sharp transition between 

 this region and the band which now looks very dark. On the other side the reflec- 

 tion revives, but more gradually, and becomes very copious in the orange and red. 

 •On this side the reflection is not technically total. If the prism is now turned so 

 that the angle of incideuce is moderate, it is found that, in spite of the equality of 

 index for the most luminous part of the spectrum, there is a pretty strong reflection 

 ■of a candle-flame, and apparently without colour. With the aid of sunlight it 

 was proved that in the reflection at moderate incidences there was no marked 

 chromatic selection, and in all probability the blackness of the band in the yellow 

 at grazing incidences is a matter of contrast only. 



