220 Mr. J. Z. Laurence on the Sensibility 



The double chlorides of manganese or magnesium with plati- 

 num differ from this composition; for their formula, according to 

 Bonsdorff*, is M CI, PtCl 2 + 6HO. 



When mixed solutions of proto chloride of cerium and iodide of 

 zinc are left for some time over chloride of calcium, a syrupy 

 mass is generally obtained ; very rarely a crystalline double salt 

 is deposited from the solution. I have not succeeded in purify- 

 ing this compound, as it attracts water with great avidity, and 

 can hardly be recrystallized in consequence of its extreme solu- 

 bility in water and alcohol. On concentrating a solution of the 

 salt in the water-bath, iodine is liberated. 



In conclusion I may mention that oxalate of cerium, lanthanium, 

 or didymium may be obtained in perfectly developed rhombohe- 

 drons, attaining often a diameter of 2 or 3 millims., when dissolved 

 in moderately concentrated nitric acid and allowed to evaporate 

 slowly over caustic lime. An acid salt, however, is not obtained in 

 this way, even when free oxalic acid is dissolved together with the 

 oxalate : for 0*7968 grm. of oxalate of cerium, dried over caus- 

 tic lime and chloride of calcium, left, on ignition, 0*3605 Ce 3 4 , 

 corresponding to 36*70 per cent, of cerium; and 1*3862 grm., 

 burnt with oxide of copper, gave 0*4784 CO 2 and 0*4175 HO, 

 corresponding to 9*41 per cent, of carbon, and 3*35 per cent, of 

 hydrogen. The formula C 4 8 Ce 2 + 8H0 requires 36*51 of 

 cerium, 9*52 of carbon, and 3*18 per cent, of hydrogen. When 

 the nitric acid is employed in a too concentrated state, and when 

 the solution is heated to ebullition, a partial decomposition takes 

 place, and a mixture of crystals of the oxalate and of free oxalic 

 acid is obtained. 



New Lodge, August 1, 1861. 



XXVI II. Some Observations on the Sensibility of the Eye to 

 Colour. By John Z. Laurence, F.R.C.S., M.B. Lond., 

 Surgeon to the South London Ophthalmic Hospitalf. 



IF, closing one eye — say the right — any highly luminous 

 white ground, such as some portions of the sky on a sunny 

 day, is viewed with the left through a dark tube so as to exclude 

 all extraneous light, after a little the eye will begin to feel 

 fatigued, and a librating circular smoky spectrum will be per- 

 ceived at the end of the tube. When the tube is laid aside and 



* Gmelin's Handbuch, vol. iii. p. 765 and 767. 



+ From the Glasgow Medical Journal, July 1, 1861. Communicated 

 by the Author. 



[Since writing this paper, my attention has been directed to a series of 

 elaborate disquisitions by Briicke and Fechner in Poggeudorff's Annalen 

 der Physik und Chemie, vols, xliv., 1., and lxxxiv., to which I beg to refer my 

 readers. — J. Z. L.] 



