150 Prof. J. 0. Bose. On the Rotation of Plane of 



other things is dependent on the direction and intensity of the 

 magnetic field, and is doubled when the ray is reflected back. 



(2) The rotation produced by saccharine and other solutions, 

 when the rotation is equal in all directions and simply proportional 

 to the quantity of active substance traversed by the ray ; the rota- 

 tion in this case is neutralised when the ray is reflected back. 



The difficulties in the way of explaining the rotation produced by 

 liquids are summarised in the following extract. 



" It is, perhaps, not surprising that crystalline substances should^ 

 011 account of some special molecular arrangement, possess rotatory 

 power, and affect the propagation of light within the mass in a 

 manner depending on the direction of transmission. The loss of this 

 power when the crystalline structure is destroyed, as when quartz is 

 fused, is consequently an event which would be naturally expected, 

 but the possession of it in all directions by fluids and solutions, in 

 which there can not be any special internal arrangement of the mass 

 of the nature of a crystalline structure, is not a thing which one 

 would have been led to expect beforehand. To Faraday it appeared 

 to be a matter of no ordinary difficulty, and I am not aware that any 

 explanation of it has ever been suggested. It is just possible that 

 the light, in traversing a solution in which the molecules are free to 

 move, may, on account of some peculiarity of structure, cause the 

 molecules to take up some special arrangement, so that the fluid 

 becomes as it were polarised by the transmission of the light, in a 

 manner somewhat analogous to that in which a fluid dielectric is 

 polarised in a field of electrostatic force."* 



In order to imitate the rotation produced by liquids like sugar 

 solutions, I made small elements or " molecules " of twisted jute, of 

 two varieties, one kind being twisted to the right (positive) and the 

 other twisted to the left (negative). I now interposed a number of, 

 say, the positive variety, end to end, between the crossed polariser 

 and analyser; this produced a restoration of the field. The same was 

 the case with the negative variety. I now mixed equal numbers of 

 the two varieties, and there was now no restoration of the field, the 

 rotation produced by one variety being counteracted by the opposite 

 rotation produced by the other. 



To get complete neutralisation, it is necessary that the element 

 should be of the same size, and that the two varieties should be 

 twisfced (in opposite directions) to the same amount. The experi- 

 ment was repeated in the following order, to avoid any uncertainty 

 due to the possible variation of the sensitiveness of the receiver. 

 The receiver is adjusted to a particular sensitiveness, and as long as 

 it is not disturbed by the action of radiation, the sensitiveness re- 

 mains constant. A mixture of opposite elements is first interposed, 

 * Preston, ' On Light,' 2nd ed., p. 421. 



