1821.] Action of Crystallized Bodies on Homogeneous Light. 115 



inside, so that the person who is logging may directly, and in 

 the very same moment, stop the line and count the knots ; the 

 watch meanwhile continues to go, and the index indicates the 

 seconds until the 28th second (for this division is adopted here 

 for the computation of our log), when the watch again gives a 

 loud stroke, and the man at that very moment stops the line, in 

 case he has let the log run to that moment which is used when 

 the ship runs at a moderate rate ; the watch is now run down, 

 and stops at once. It is not wound up in any other manner : 

 this is effected by the pressure upon the spring by which it 

 repeatedly may be set going. 



The contrivance appears to be somewhat superior to that 

 described by Mr. Newman. Mr. Sparrovogn (the above-men- 

 tioned Danish watch-maker, who is now dead) made several of 

 these log-watches. The Danish Board of Admiralty rewarded Mr. 

 Sparrevogn with a proper remuneration, and Admiral Lowenom 

 laid before the Royal Society of Sciences at Copenhagen (of 

 which he is a Fellow) a drawing of the construction of this watch, 

 together with an explanatory descriptioi: of it, which met the 

 approbation of the Society. 



Article IX. 



On the Action of Crystallized Bodies on Homogeneous Light, and 

 on the Causes of the Deviation from Newton's Scale in the 

 Tints which many of them develope on Exposure to a polarised 

 Ray. By J. F. W. Herschel, Esq. FRS. Lond. and Edin.* 



Since the period of the briUiant discovery of Malus of the 

 polarisation of light by reflection, the investigation of the gene- 

 ral laws which regulate the action of crystallized bodies on light 

 has advanced with a rapidity truly astonishing, and the labours 

 of an Arago, a Brewster, and a Biot, have already gone far 

 towards completing the edifice of which that distinguished phi- 

 losopher laid the foundation. When Malus wrote, the list of 

 doubly refracting crystals was small, and the most remarkable 

 among them possessing only one axis of double refraction, it 

 seems to have been for some time, tacitly at least, presumed that 

 the law discovered by Huygens, and since re-established in the 

 most rigorous manner for that one,t might hold good in all. 



* Read before the Royal Society of London, Dec. 23, 1819- 



i' The author of the article on Polarisation, in the 6'jd number of the Edinburgh 

 Review, just published, is guilty of a most unpardonable mistake, in asserting (p. 188), 

 as deducible from Dr. Brewster's experiments, that the Huygenian law is incorrect^ for 

 carbonate of lime. Dr. Brewster's general formula for crystals with two axes resolve 

 themselves into the Huygenian law when the axes coincide, of which case it is only an 

 extension. That excellent philosopher, if I understand English, in the paragraph 

 "which gave rise to this strange' assertion, only means to declare his opinion that it 

 remains undemonstrated. 



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