286 
Dr. Brewster on the optical 
extremely low, and to vary in different specimens, as stated 
in the following table. 
Index of Refraction. 
Transparent tabasheer from Vellore* 
1.1115 
Transparent tabasheer from Nagpore 
1.1 454 
Another specimen of the same 
1*1503 
A third specimen of the same 
1*1535 
A harder and more opaque specimen, 
1.1825 
Water 
1*3358 
Flint glass - - - 
1.600 
Sulphur - 
2.115 
Phosphorus - 
2.224 
Diamond - 
2.470 
Hence it follows, that tabasheer has a lower refractive power 
than all other bodies, whether solid or fluid ; and that it 
holds an intermediate place between water and the gases . 
This extraordinary result which, as it were, insulates taba- 
sheer from all known substances, will enable us to give an 
explanation of some of its most remarkable properties. 
The singular nature of this substance will appear in a 
still more striking point of view, by comparing its absolute 
refracting power with that of other bodies. The low refrac- 
tive power of air, when contrasted with that of water, and 
of water when contrasted with that of solid bodies, obviously 
arises from the great difference of their specific gravities, and 
not from any peculiar action upon light. If we call R the 
* This specimen, with which I was favoured by Dr. Hope, formed part of the 
tabasheer sent by Dr. Patrick Russell to the Royal Society, in 1790. It was 
yellowish by reflected light, and so extremely tender, that I was obliged to polish it 
upon the softest silk. 
