$60 Prof. Jellett on a new Optical Saccharometer. 



in 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. But this is not true of the planes of po- 

 larization 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 prepon- 

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

 light, when the densities of the two parts are equal. The differ- 

 ence 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 result, arising from an incorrect position of the tube, is in- 

 versely proportional to the length of the column of the compen- 

 sating 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. 



No general rule can be given for determining the strength of 

 the solution 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 pro- 

 portional 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 examination 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 con- 

 taining in each cubic inch thirty grains of sugar may be used 

 without inconvenience*. 



* The instrument here described was constructed by Messrs. Spencer 

 and Son, of Aungier Street, Dublin, 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. 



