1899-1900.] Mr J. J. Manley on Examination of Sea- Water. 35 
The Examination of Sea- Water by an Optical Method. 
By J. J. Manley, Magdalen College Laboratory, Oxford. 
Communicated bij Sir John Murray, K.C.B. 
(Read January 8, 1900). 
In a paper* communicated to the Royal Society, Mr R. T. 
Gunther and the author gave an account of the results ob- 
tained from the examination of two samples of water taken from 
Lake Urmi, and amongst other determinations of a chemical and 
physical nature, were those of the refractive indices, which were 
performed with the aid of the Royal Society’s large quartz prism 
and spectrometer, the latter reading by means of micrometers to 2" 
of arc. On comparing the values obtained for the refractive indices 
of the two samples of water with those obtained for the relative 
densities, it was at once apparent that the former differentiated 
the two samples quite as distinctly as the latter. 
Krummelf attempted an optical method for the examination 
of various samples of sea-water, by measuring their refractive 
indices with the aid of an Abbe refractometer. The chief objec- 
tions to the use of this instrument are — (1) Its sensibility is not 
sufficient when the waters to be examined differ but slightly from 
each other in their degrees of salinity ; (2) the drop of water placed 
upon the fixed prism must necessarily undergo a certain although 
small amount of evaporation before it can be covered by the second 
or movable prism ; (3) there is a considerable degree of uncertainty 
as to the true temperature of the liquid contained between the 
prisms, even when the refractometer is supplied with a water 
jacket. The thermometer indicates the temperature of the water in 
the jacket, but, owing to the unavoidable massiveness of the 
prisms, and the bad conducting power of glass for heat, it is highly 
improbable that the temperature observed is also that of the 
liquid whose refractive index is being measured. 
* Proc. Roy. Soc., vol. 65, 1899, p. 312. 
t Annalen der Hydrographic, 1894, p. 241. 
