508 Mr. R. H. M. Bosanquet on the 



effect is perfectly in accordance with all the other experiments ; 

 and the hypothesis which explains them, whatever it may be, will 

 also explain this. 



We have seen that, with three kinds of smoke, the variation 

 of the stage of advancement of the action is of a remarkably 

 limited character. With the incense and gunpowder the 

 action was entirely within Stage II., and the greatest variation 

 of the neutral angle was only 33° in the first experiment and 

 25° in the second. It was only with the fumes of resin, "ren- 

 dered so dense as to be very irritating to the lungs/' that 

 Stage III. was entered upon, and this, according to the descrip- 

 tion, by only a little step. This appears to indicate a certain 

 uniformity in the size of the particles which constitute the smoke 

 employed ; and, at all events, the inference may be safely drawn 

 that smoke produced in any ordinary way is not likely to proceed 

 so far as the end of Stage II. Now what happened was that the 

 track of the beam in ordinary air was observed, not with a polari- 

 meter, which would have given some sort of measure of the ratio 

 of polarized to common light, but with a selenite and Nicol, a 

 combination which gives no measure of the kind. For instance, 

 supposing light to contain half polarized and half common light. 

 Then, if the light be very faint, it will give with selenite and 

 Nicol a faint ring-system ; if the light be stronger, the ring- 

 system will be stronger; in short, the indication depends as 

 much on the strength of the light as on its percentage compo- 

 sition. The indication of the polarimeter, on the contrary, is, 

 or ought to be, independent of the strength of the light. 

 Suppose, then, that into the faint luminosity of the beam in air a 

 puff of smoke is thrown in a state corresponding to the initial 

 condition of experiments 1 or 2. The normal emission is greatly 

 increased, for there is so much more matter to disperse the light ; 

 and the polarization is still positive, for the state of the smoke is 

 still below Stage III. There is, then, so far no need for additional 

 explanation ; the increased light amply explains the increased 

 brilliancy of the ring-system. 



When vapour is employed, it needs little consideration to see 

 that the particles may be of any size. In fact, in the experiments 

 with tubes, it was the natural course for them to begin, in Stage 

 I., much smaller than the smoke particles ever are, and, passing 

 rapidly through Stage II., to indicate by the negative polarization 

 emitted normally, as well as by their coarser physical structure, 

 their rapid growth in size. 



W T hat seems to make it clear that the transparency is at all 

 events not a necessary condition for Stage III. is, that it was 

 attained with the resin smoke, the particles of which were no 

 doubt opaque. 



