281 



But the author has found that in certain cases the error is very percep- 

 tible indeed. This is shown hj the impossibility of giving to the tube 

 ff any position m which the shades of colour are exactly the same in the 

 two parts of the circular image. Suppose, for example, that the position 

 of the tube is such that the plane of polarization of the mean ray has the 

 same position as at first. This plane is then equally inclined to the 

 planes of analyzation of the two parts of the analyzing prism. Eutthis 

 is not trae of the planes of polarization of any of the other rays ; of 

 these, the less refrangible will have their planes of polarization nearer 

 to one of the planes of analyzation, while those of the more refrangible 

 are nearer to the other. 



There will therefore be in the one half of the image a preponderance 

 of red light, and in the other a preponderance of blue light, when the 

 intensities of the two parts are equal. The difference of colour, which 

 makes it difficult to equalize these intensities with perfect accuracy, 

 will evidently be greater, the greater the amount of the rotations which 

 the compensating fluids would severally produce, and therefore the 

 greater the strength of the solution. 



On the other hand, it must be remembered that the error in the re- 

 sult, arising from an incorrect position of the tube, is inversely propor- 

 tional to the length of the column of the compensating fluid. Thus, if 

 the reading of the scale be '1, an error of one division, or -001 will have 

 the same effect on the result, as an error ten times as great would have, 

 if the reading were 1*000. 



Ko general rule can be given for determining the strength of the so- 

 lution which it is desirable to use. If the law of Biot, sc., — that the 

 amounts of rotation produced by the same substance in the planes of 

 polarization of the different simple rays are proportional to the squares 

 of the corresponding refractive indices — be strictly true, then, the more 

 nearly these indices are in the same proportion for the fluid under exa- 

 mination and the compensating fluid, the stronger may be the solution 

 used. If the fluid under examination be a saccharine solution, and the 

 compensating fluid French oil of turpentine, a solution containing, in 

 each cubic inch, thirty grains of sugar, may be used without inconve- 

 nience.^' 



James Dombrain, Esq., of Monkstown, through Gilbert Sanders, 

 Esq., presented a very perfect long tapering sword-blade, made of bronze, 

 found in a bog, near Timoleague, county of Cork. 



Henry Kingsmill, Esq., on the part of his son, Henry Kingsmill, Jun., 

 Esq., presented a collection of rubbings from monumental brasses. 



The Master of the EoUs in England, through the Librarian, pre- 

 sented a large collection of Becord publications, completing the series 

 already in the Library of the Academy, and consisting of 63 volumes. 



The thanks of the Academy were presented to the donors. 



* The instrument here described was constructed by Messrs. Spencer and Son, of 

 Aungier-street, to whose ability, both in carrying out the instructions given to them, and 

 in suggesting methods for overcoming practical difficulties, the author is much indebted. 



