470 Proceedings of the Royal Society 
moment a more refractive medium than the spar, and therefore it 
suffers that image to pass alone.” 
The preceding remarks were published in the year 1834. Soon 
afterwards I perceived that if my explanation were correct, a Nicol 
prism might be made, half of calc spar and half of glass. Theory 
indicated this, but no actual experiment of the kind was made at 
that time. Recently, however, my attention has been once more 
directed to this subject, and I have had such an instrument con- 
structed by Mr Bryson, optician, of Edinburgh, with a very satis- 
factory result. When light has been polarised by an ordinary 
Nicol prism, it is completely extinguished by the new prism held 
in a proper position; whereas when two Nicol prisms are com- 
bined, a small portion of light generally remains visible. 
Either end of the new prism may be held foremost, a result 
which was not altogether expected. An idea is prevalent that the 
action of an ordinary Nicol prism is due to the circumstance that 
one surface of the calc spar is left rough to scatter one of the rays. 
But such is not the case. Both surfaces are highly polished by the 
best makers, and the ray is not scattered, but reflected, and maybe 
seen by proper management. 
3. Note on a New Scotch Acidulous Chalybeate Mineral 
Water. By Janies Dewar, F.RS.E. 
It is generally known that this country is extremely deficient 
in well-marked chalybeate waters. Plenty natural waters, con- 
taining small proportions of iron, are to be met with in the 
United Kingdom ; but, with the exception of those of Tun- 
bridge Wells, Harrogate, Sandrock (Isle of Wight), Heartfell, 
near Moffat, and Vicarsbridge, in the vicinity of Dollar, they con- 
trast very unfavourably with those of the numerous spas of the 
continent of Europe. If we restrict ourselves to an examination 
of the chemical characters of the above-mentioned Scotch chaly- 
beates, we observe that the iron is present in large quantities in 
the form of sulphate, along with sulphate of alumina, on which 
account they are more nauseous to invalids, and are at the present 
time rather unpopular. 
Recently my brother, Dr Alexander Dewar, Melrose, sent me for 
