TRANSACTIONS OF THK SECTIONS. 41 



is not constant ; some of them see three only, others four (among which blue and 

 red may be specially mentioned). The extreme colours, red and violet, are frequently 

 not distinct to them. This latter fact appears to me to be intimately connected with 

 the question as to the number of primary colours {couleurs elementaires) , 



5. The degree of polish of the coloured surface exerts an influence on the appre- 

 ciation of its colour. 



6. Some Daltonians perceive both the brightness and the coloration of comple- 

 mentary tints which are invisible to us. 



7. Two colours, which appear to our eyes to pass into each other by a succession 

 of intermediate tints, appear to them [Daltonians] in contrast with one another. 



8. Daltonians see exactly as we do the black rays [spaces] discovered in the spec- 

 trum by Frauenhijfer, at least in that portion of the spectrum which to them appears 

 illuminated. 



9. Those colours which we call complementary do not always appear such to them ; 

 and conversely, the tints which they designate as complementary to those which they 

 perceive, are not such to our vision. 



It is remarkable that the subject of Daltonism appears to have been studied by the 

 English only ; for, with the exception of the works of Dr. Jiincher and Prof. Seebeck,' 

 it is difficult to refer to any researches on this subject but those of Englishmen. I shall 

 be happy if by this notice I shall have excited physiologists and physicians to make 

 new observations on the subject, and I shall receive with acknowledgement any which 

 may be communicated to me. 



On the application of a Coating of Gold hy the Electro-metallurgic process to 

 the Steel Balance-springs of Chronometers. By E. J. Dent. 



It is customary to " blue the balance-spring " in chronometers, and the blue oxy- 

 genated surface (or coating) greatly increases the elastic force of the spring ; on its 

 removal the balance-spring suffers a nearly corresponding loss. This rigid oxyge- 

 nated coating, on its first formation by heat, increases the strength of the balance- 

 spring more than the additional application of the gold on its surface ; and while 

 it may be considered as a first process of rust, the gold surface is a protection from 

 the ill effects of damp and saline atmospheres, to which chronometers are subject on 

 ship-board, and particularly in tropical climates. Mr. Dent first applied the gold to 

 a chronometer balance-spring which had been previously rated ; on its being replaced 

 in the chronometer he found the rate of the chronometer to be losing 4 1 seconds ia 

 24 hours, which was caused by the removal of the blue oxygenated surface, and the 

 gold coating not increasing the elastic force so as to compensate for the loss. Mr. 

 Dent in another paper communicated the result of his experiments on the glass ba- 

 lance-spring in chronometers, since his first communication made to the Association 

 at Cambridge in 1833, with observations on its official rate, resulting from five years* 

 trial by order of the Lords Commissioners of the Admiralty. 



On the Preservation of Magnetic Needles and Bars from Oxidation by the 

 Electrotype process. By Professor Christie. 



The author stated that the preservation of the identity of magnetic needles and bars 

 employed in determining the terrestrial magnetic intensity, whether statically or dy- 

 namically, both as regards their magnetism and their weight, is so evident, that it 

 was quite unnecessary for him to dwell upon the subject ; but he might mention, that 

 even in the case of the ruder instruments employed— as ships' compasses, it had been 

 considered by experienced naval officers that it would be advantageous if the needles 

 were efficiently protected from oxidation. On learning that the electrotype process 

 had been applied by Mr. Dent to the protection of the balance-springs of chrono- 

 meters, by forming a coat of pure gold on their surface, it occurred to him that 

 the same process was admirably adapted for protecting magnetic bars and needles 

 from oxidation. He now presented to the Section two needles, which, after having 

 been magnetized, were coated with gold by this process. He considered that the 

 same process could be advantageously applied to the protection of the axes of dip- 

 ping needles : this however was a question which could only be decided by careful 

 experiment, as it was very possible that the coating of soft gold on the axis might 



