in the Neighbourhood of the Polarizing Angle. 17 



Band achromatic. 



+ 30° 

 -30° 



180° 2' 

 180° 54' 



+ 30° 

 -30° 



180° 47' 

 179° 59' 



Band strongly coloured 

 red above. 



The difference of readings is here about —50', giving 

 It— — *0043. On trial it was found that camphor fragments 

 would just move. The above is an example of the effect of 

 the position of the mica upon the coloration of the band, a 

 subject already discussed. 



Experiments were made with the object of comparing 

 different kinds of oil as to their relative effects, optically and 

 upon camphor. It was found, as had been expected,, that 

 cassia was more powerful optically than olive-oil. Thus when 

 camphor was nearly dead the difference of readings for olive- 

 oil was about —30' and for cassia about —48'. 



Interest was felt in the behaviour of a saturated solution of 

 camphor, whose surface-tension is much lower than that of 

 clean water. Observations upon this liquid proved especially 

 difficult, for the dark spot frequently shifted laterally while 

 under inspection, indicating temporary changes in the ellipti- 

 city of the particular part of the surface in use. There is 

 little doubt that this complication is due to local evaporation 

 under the influence of light currents of air. As the camphor 

 evaporates from any part of the surface, the tension is momen- 

 tarily raised, and the surface contracts. If the camphor only 

 were in question, there would probably be no attending 

 optical disturbance, but the local expansions and contractions 

 of the surface lead to attenuation and concentration of the 

 greasy matter present. Under favourable circumstances the 

 difference in the readings of x might be as low (numerically) 

 as — 6', and was perhaps due after all to residual greasy matter, 

 other than camphor. In any case the optical effect of the 

 camphor is much less than that of an oily film giving the 

 same surface-tension. 



With a strong solution of oleate of soda the difference of a 

 could not be reduced below —25'. It is difficult to suppose 

 that this can be due to a film of foreign matter removable by 

 skimming. But the amount of the ellipticity is very low in 

 relation to the surface-tension, which is only about one third 

 of that of clean water. The value of k corresponding to the 

 above readings is —'0021, only about double of the smallest 

 quantity appreciated by Jamin. 



That the surface-tension has no definite relation to the 

 Phil. Maq. 8. 5. Yol. 33. No. 200. Jan. 1892. C 



