96 Lord Rayleigh on the 



by a large lens of 2 or 3 feet focus; and in the path of the light 

 was placed a beaker glass, containing a dilute solution of hy- 

 posulphite of soda. On the addition of a small quantity of 

 dilute sulphuric acid a precipitate of sulphur slowly forms, 

 and during its growth manifests exceedingly well the pheno- 

 mena under consideration. The more dilute the solutions, the 

 slower is the progress of the precipitation. A strength such 

 that there is a delay of four or five minutes before any effect 

 is apparent, will be found suitable; but no great nicety of ad- 

 justment is necessary. By addition of ammonia in sufficient 

 quantity to neutralize the acid, the precipitation may be 

 arrested at any desired stage. More time is thus obtained to 

 complete the examination ; but the condition of things is not 

 absolutely permanent, the already precipitated sulphur appear- 

 ing to aggregate itself into large masses. 



In the optical examination we may, if we prefer it, polarize 

 the primary light; but it is usually more convenient to analyze 

 the scattered light. In the early stages of the precipitation 

 the polarization is complete in a perpendicular direction, and 

 incomplete in oblique directions. After an interval the pola- 

 rization begins to be incomplete in the perpendicular direc- 

 tion, the light which reaches the eye when the nicol is in the 

 position of minimum transmission being of a beautiful blue, 

 much richer than any thing that can be be seen in the earlier 

 stages. This is the moment to examine whether there is a 

 more complete polarization in a direction somewhat oblique ; 

 and it is found that with 6 positive there is in fact an oblique 

 direction of more complete polarization, while with negative 

 the polarization is more imperfect than in the perpendicular 

 direction itself. 



The polarization in a distinctly oblique direction, however, 

 is not perfect, a feature for which more than one reason 

 may be put forward. In the first place, with a given size of 

 particles, the direction of complete polarization indicated by 

 (61) is a function of the colour of the light, the value of 6 

 being three or four times as large for the violet as for the red 

 end of the spectrum. The experiment is, in fact, much im- 

 proved by passing the primary light through a coloured glass 

 held in the window-shutter. Not only is the oblique direction 

 of maximum polarization more definite and the polarization 

 itself more complete, but the observation is easier than with 

 white light, by the uniformity of the colour of the light scat- 

 tered in various directions. If we begin with a blue glass, we 

 may observe the gradually increasing obliquity of the direction 

 of maximum polarization; and then by exchanging the blue 



