Mr. A. B. Northcote on the Brine-springs of Cheshire. 457 
Since the critical angle for the ordinary ray is 37° 12’, and 
when the ordinary light commences to undergo total reflexion 
the extraordinary ray makes an angle of incidence 37° 12'—6° 40! 
=30° 32!, we have the range between this and 39° 32! within 
the crystal giving a pencil of the angle 9°, which is sufficient to 
furnish conveniently a broad beam of polarized light. From 
the nature of the double refraction, the angular separation of 
the ordinary and extraordinary rays diminishes as they approach 
the optic axis, and they coincide when in that direction; and 
hence the incident and emergent beams, when both the prisms 
are of cale-spar and similar, subtend angles of only about 8° to 9°. 
When one of the prisms is of glass, we have other properties 
to consider, and have different results as the glass or spar is turned 
towards the eye, when the rhomb is used as an analyser. When 
the spar is cut along the optic axis, and the light is incident 
nearly perpendicular to the face so produced, the two rays coin- 
cide in direction, but have the greatest difference of refractive 
indices, that of the extraordinary ray being somewhat less than 
the one for common plate glass ; a beam passing into a compound 
rhomb in this manner, we may have the extraordinary ray under- 
going total reflexion at the last surface of the glass or that nearest 
the eye; and so an angular magnitude as well as breadth of pencil 
may be procured which will render the rhomb serviceable both 
as polarizer and analyser where Nicol’s rhomb and the double- 
image rhomb would be much less applicable. When the experi- 
ments which I have in hand with this form of rhomb are com- 
pleted, I intend to give an account of them in another paper. 
I have already obtained pencils of about 11° polarized in one 
plane with the ordinary rays transmitted. 
London, November 7, 1857. 
LUI. On the Brine-springs of Cheshire. By Augustus Breav- 
cuamp Nortucore, /.C.S., Senior Assistant in the Royal 
College of Chemistry *. 
° ale existence of salt deposits in Cheshire, and of the brine- 
springs which flow from them, has from the earliest historic 
periods exercised a very peculiar influence upon the economic 
features of that county. It appears from the record of Domes- 
day, that salt was even then one of the principal articles of its 
commerce, and that at a period anterior to the Norman Conquest 
it brought in a considerable revenue to the Crown; for as early 
as the time of Edward the Confessor, the Wiches, as they were 
called, are stated to have been very productive, and the tolls 
levied upon the amount of salt sold were divided in the pro- 
* Communicated by the Author. 
