C. GENERAL PHYSICS. 163 



THE FORMATION OF "GLORIES." 



If one contemplates his shadow in full sunshine as thrown 

 upon any rough surface, especially upon the dew-drops cov- 

 ering: a field of grass, the shadow of the head will be seen 

 surrounded by a feeble halo of light, which ordinarily extends 

 above the head further than on either side. Of several ob- 

 servers standing side by side, of course each one sees only 

 his own shadow surrounded by this glory. Similar phenom- 

 ena have been observed by aeronauts when their shadows 

 are thrown down upon the upper surface of the clouds. The 

 explanation of these phenomena has been sought by Lom- 

 mel, who concludes that the "glory" consists of rays of 

 light, which, entering into the drops of dew or fog or rain, 

 are refracted and reflected from the further surface of the 

 drop, and finally return to the source of light. These rays 

 have, therefore, suffered one reflection and four refractions; 

 in other words, the phenomena are caused in the same way 

 as the shining of the eyes of cats and other animals which 

 have a strongly reflecting surface at the back of the eye. In 

 case of large rain-drops, the phenomenon appears only when 

 these lie, like dew-drops, upon the surface of the leaves, and, 

 by virtue of their rounded form, act like small lenses. 19 C, 

 1874,146. 



EXPANSION OF EBONITE BY HEAT. 



Kohlrausch, having accidentally observed that ebonite lids 

 stick fast in glass vessels, suspected that this material might 

 have a considerable expansibility by heat, and his expecta- 

 tion has been realized by finding that it is about three times 

 as expansible as zinc. Tins great expansion may possibly be 

 connected with the proportion of sulphur which - ebonite con- 

 tains. On the other hand, the contrast with soft caoutchouc 

 is remarkable. The increase of the co-efficient of expansion 

 with temperature is very considerable. One fact is mention- 

 ed relating to the expansion which seems to be of peculiar 

 value. The bar of ebonite, which was about a centimeter in 

 thickness, after being heated, required some time before it 

 assumed a constant length. Although the bad conductivity 

 is doubtless the principal cause of this, the author thinks that 

 another agent also is at work. Like the elastic change of 



