Royal Society, 51 



B being the point of view, that colour which is observed about 

 BC B'C' alone, not in the direction of the axis, must have its 

 vibrations j»flra//e/ to AA' or to CC, certainly wo^ jo«ra//e/ to BB', 

 for these would be longitudinal. The point of view being C, 

 the vibrations would, it is equally certain, be parallel to AA' or 

 parallel to BB', certainly not parallel to CG', for these again 

 would be longitudinal. To one colour, however, belongs only one 

 direction of vibration ; BB' and CC are both excluded, and A A' 

 remains as the only possible direction of vibration. This stands 

 perpendicular to the plane BC BfO, But this is polarized per- 

 pendicular to the axis, hence the direction of vibration is perpen- 

 dicular to the plane of polarization. 



XII. Proceedings of Learned Societies, 



ROYAL SOCIETY. 



[Continued from vol. iv. p. 476.] 

 June lO/'tj^XPERIMENTS towards the construction of new forms 

 1852, -^ of Instruments for the correction of Compass Errors 

 due to the presence of iron in ships ; with investigations on the na- 

 ture of the attraction of Iron on the poles of Magnets." By Julius 

 Roberts, Esq., Lieut. R.M. Artillery. Communicated by Capt. W. 

 H. Smyth, R.N., For. Sec. R.S. 



The object of the author's experiments and investigations is stated 

 to have been the production of an instrument that would, under all 

 variety of circumstances, give a correct (magnetic) meridian direc- 

 tion, or in some way indicate the amount of its own error ; and he 

 considers that he has, at least partially, succeeded in the attainment 

 of this object in the instrument described in this paper, and of 

 which drawings accompany the communication. 



In order to determine the nature of the action of a mass of iron on 

 a magnetic needle, the author constructed needles with the magnetic 

 bar wholly on one side of the central support, counterpoised by an 

 arm carrying a weight on the other. In some of these the magnetic 

 bar was straight, in others it was bent in the middle, either the 

 upper or lower half being horizontal. He also constructed a com- 

 pound needle of two such bent bars, suspended concentrically with 

 two of their contrary poles remote, the other two poles being the 

 one above and the other below the points of support. In order to 

 give magnetic stability to the compound needle, an arc of soft iron 

 was attached to the extremity of the counterpoising arm of each 

 magnet, so as to be in close proximity, though not actually to touch 

 the other. Attached to the counterpoise arms were the halves of a 

 compass-card cut through the east and west points, and so adjusted 

 that the edge of one was vertically over that of the other when the 

 needles were in the same vertical plane, but crossed each other when 

 the needles deviated from the same direction. The author con- 

 sidered that a mass of iron attracting each pole of this compound 

 needle would cause the cards to cross each other, and thus give the 



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