﻿276 The Hon. J. W. Stmtt on the Light from the Sky, 



mitting that if the particles are very small the ordinary laws of 

 reflection and refraction no longer apply - *, and that therefore 

 this case is not excluded by his argument, he still holds to his 

 original view as to the nature of the reflecting matter in the sky, 

 considering that the polarization of the light indicates -that it 

 has undergone regular reflection. His concluding paragraph so 

 well sums up the case that I cannot do better than quote it. 

 " Das Resultat der vorstehenden ^Betrachtungen kann ich hier- 

 nach kurz so zusammenfassen. Soweit man die gewohnlichen 

 Brechungs- und Reflexionsgesetze als giiltig anerkennt, glaube 

 ich auch meine fruheren Schliisse festhalten zu miissen, namlich, 

 dass in der Atmosphare Dampfblaschen vorhanden seven, und 

 dass sie die Hauptursache der in ihr stattfindenden Lichtreflexion 

 und ihrer Farben bilden. Nimmt man aber an, die in der 

 Atmosphare wirksamen Korperchen seyen so klein, dass jene 

 Gesetze auf sie keine Anwendung mehr linden, dann sind audi 

 diese Schliisse ungiiltig. Auf diesen Fall ist aber auch die Theorie 

 der Farben dunner Blattchen nicht mehr anwendbar, und er 

 bedarf vielmehr einer neuen Entwickelung, bei welcher noch be- 

 sonders benicksichtigt werden muss, in wiefern diese Annahme 

 mit der Polarisation des vom Himmel kommenden Lichtes und 

 mit der angenahert bekannten Grosse der in den Wolken vor- 

 handenen Wassertheilchen vereinbar ist." 



Clausius does not seem to have followed up the line of research 

 here indicated. My investigation (written, it so happens, be- 

 fore seeing Clausius's papers) shows in the clearest manner the 

 connexion between the smallness of the particles and the polari- 

 zation of the light scattered from them. Indeed I must remark 

 that in this respect there is an advantage over the theory of thin 

 plates, according to which the direction of complete polarization 

 would be about 76° from the sun. It would be a singular coin- 

 cidence if the action of secondary causes were to augment this 

 angle to 90° — its observed magnitude. It seems, therefore, not 

 too much to say that, if the illumination of the sky were due to 

 suspended water-bubbles, neither its colour nor its polarization 

 would agree with what is actually observed. 



In his celebrated paper on Fluorescence f, Professor Stokes 

 makes the following significant remark : — " Now this result ap- 

 pears to me to have no remote bearing on the question of the 

 direction of the vibrations in polarized light. So long as the 



* In many departments of science a tendency may be observed to extend 

 the field of familiar laws beyond their proper limits. Thus the properties 

 of gross matter are often assumed to hold equally good for molecules. An 

 example more analogous to that which suggests this remark is to be found 

 in the common explanation of the mode of action of the speaking-trumpet, 



t Phil. Trans. 1852, p. 526. 



