TRANSACTIONS OF SECTION A. 563 



The method of experiment was to adjust the gilt kuohs of the oscillatois about 

 5mm. apart, and, starting- from the most distant end of the wire, piclc out nodes by 

 careful adjustment of the length of the spark gap in the resonating circle. Two 

 internodal distances were usuall}' measured, but sometimes three or four. 



The lengths of the internodal distances were found to agree well for the same 

 wire and receiver. The mean of thirteen measurements with a wire 4'9 metres 

 long and l'57mm. diameter gave a wave-length of 0-605 metre. Five experimentd 

 with a wire 4 metres long and 2-9mm. diameter showed the wave-length 0-05 metre. 

 A brass gas-pipe was next tried. This was 3-G metres long and 11mm. diameter 

 and gave a wave-length of 0*62 metre, as a mean of fourteen experiments. 



A thin wire, 5 metres long andO'36mm. diameter, gave a wa\e-length of 0-476 

 metre (mean often experiments). 



A very fine wire, 4 metres long and 078mm. diameter, gave as a mean of 

 twelve experiments a wave-length of 0-384 metre. 



The author thinks that these experiments show that Professor Hertz's residts 

 ■were due to the comparative thinness of the wire he used as judged by the length 

 of his waves. 



7. On Reflection near the Polarising Angle from the Clean Surfaces of 

 Liquids. By Lord Eayleigh, Sec.B.S. 



If the image of the sun, reflected at the polarising angle from the surface of 

 ordinary water, be examined through a good nicol, no complete extinction can be 

 observed. At most a dark nebulous patch is seen upon the face of the sun. If, 

 however, the surface of the water be i'ree from contamination, a well-detined band 

 crosses the solar disc, coloured above and below, and to all appearance black, or 

 nearlj' so, at its centre. The width of this band may be estimated at about one- 

 fifth of the solar diameter. A trace of olive-oil, decidedly short of what is 

 required to stop the camphor movements, practically obliterates the band. The 

 colour seen from clean water, which is due to the variation of the polarising 

 angle with wave-length, may be compensated by holding a 20° water prism 

 between the eye and the nicol. The band is thus achromatised, but colour is of 

 course introduced at the upper and lower limbs of the sun. 



The deterioration of the band by contamination is not measured by the decre- 

 ment of surface tension. A strong solution of oleate of soda or a saturated solution 

 of camphor may give a much better band than distilled water with a somewhat 

 greasy lace. Moreover, difl'erent parts of the same surface (over which the tension 

 is constant) are often observed to produce very different effects. 



Preci.'e measures of the ellipticity abundantly confirm these preliminary 

 results. Sunlight passing through a round hole fitted with cross-wires falls upon a 

 collimatiug lens, thence after reflection from an adjustable mirror traverses the 

 polarising nicol, and after reflection from the horizontal liquid surface passes a 

 quarter-wave mica and an analj-sing nicol. The latter is set alternately to a 

 deviation of ± 30° from the plane of incidence, and the azimuth of the polariser 

 required to bring the dark spot upon the cross- wires in each case is recorded. 

 If 2a be the diflerence of readintrs, tan 30° tan a, denoted by k, represents the 

 ellipticity, measuring as it does the ratio of reflection of amplitudes of the two 

 ])rincipal components. Jamiu found for water k=— -00577, and for alcohol 

 /c = + -00208. 



In my apparatus, which worked remarkably well, a change of setting of the 

 polariser of about two minutes was directly apparent when the analyser stood at 

 ± 30°, and very early experiments showed that the ellipticity of clean water could 

 barely be measured. While in the ordinary water 2a might lie between ^°and 1^°, 

 the value for clean water seemed not to exceed 2'. Usually no error could be 

 perceived by mere inspection when the analyser was put over from + 30° to —30°; 

 and the meiin of long series of alternations gave a diflerence sometimes in one 

 direction and sometimes in tlie opposite. These discrepancies could only be 

 attributed to real changes in the purity of the surfaces, and evidence gradually 

 accumulated that the value for a clean surface was not zero, as had been expected, 



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