of Edinburgh , Session 1877-78 
569 
physical collections in Wurtzburg and Heidelberg, the refractive 
index for the Fraunhofer line D varied between 1*5141 and 1*5374 
for the ordinary ray, and between 1*5216 and 1*5470 for the extra- 
ordinary. 
I allow the first surface of the quartz to adhere by capillary 
attraction, and my large horizontal circle, by which /x and i are 
measured, reads to two seconds. 
Crown glass plates from Steinheil, which had lain ten or twelve 
years in a press, and whose refractive index for D was 1*5245, gave, 
by total reflection at the surface, the refractive index 1*4903. 
The alteration of the surface appears to me to be due, in quartz as 
in glass, to a chemical change of surface, perhaps to the vapour in 
the air forming a hydrate of silicic acid, or a hydrate of silic. 
The above measures and arrangements have not yet been pub- 
lished, but they are entirely at your service if you can use them in 
your article on Elasticity. 
I hoped to have sent you with the former ones some measures of 
the refractive indices of natural quartz surfaces, but then the obser- 
vations have to be made with reflected light, which impairs their 
accuracy; besides, in spite of great trouble, I have not been able to 
procure any quartz whose surfaces were flat enough and complete 
enough for this inquiry. The only crystalline surface which I 
could examine seemed to have the same refractive indices as fresh 
polished surfaces. Besides, I am not astonished to find different 
refractive indices in different quartz crystals, since I have invariably 
found slight variations in the optical constants even for light trans- 
mitted through different specimens of crystals, for instance, in the 
amount of rotation of the plane of polarisation. Even if one had 
kept the crystal for thousands of years under the same physical 
conditions, for instance, at the same temperature, &c., still, accord- 
ing to my opinion, the mode in which it was originally formed 
would affect its final stationary condition. Crystals, like human 
beings, and like films of liquid upon heterogeneous solid or liquid 
surfaces, carry with them during their whole existence the mark of 
their origin or birth. Two bodies can only show properties ex- 
tremely alike, never exactly the same. 
