280 



through a transverse slit in the outer tube, so that when partly un- 

 screwed they allow the prisms to turn through a small angle round 

 the axes of the tube. In using the instrument, the polarizing prism 

 may be set in any position, the analyzing prism being then carefully ad- 

 justed, so that the tints in the two halves of the circular spectrum* 

 may, when there in no fluid interposed, be exactly equal. 



Suppose now that the object is to ascertain the strength of a given 

 solution of cane sugar. In this case, the fluid to be used in the vessel, 

 jEE, may be Trench oil of turpentine. A certain quantity, the amount 

 of which depends on the strength of the solution to be observed, having 

 been poured into the vessel, the tube, ff, is then filled with a solution of 

 sugar, whose strength is accurately known. The tube is now replaced 

 in the upright pieces, and the zero of the vernier made to coincide ac- 

 curately with the zero of the scale. The milled head is now turned so 

 as to draw back the tube until the tints on the two parts of the circular 

 image, seen through L, become equal. The number on the scale cor- 

 responding to the zero of the vernier is then noted. Let this reading be 

 R, and let S be the strength of the known solution. 



E'ow, let this solution be removed from the tube, which is then to 

 be filled with the solution whose strength is required. The same pro- 

 cess having been gone through, let the new reading be ; then the 

 strength required is given by the equation — 



If the experiment be carefully conducted, and if there be no error in 

 the strength of the standard solution, the error in the measurement 

 made, as above described, ought not to exceed 0 grs. '02 per cubic inch 

 for a single experiment. If the mean of a number of experiments be 

 taken, the error would, of course, be still less. 



The author has given to this instrument the name saccharometer, 

 derived from one important use to which it may be applied. This, how- 

 ever, is but one of its applications ; and there are many others, at least 

 as important. It may generally be defined to be an instrument by 

 which the ratio of the rotatory power of any transparent fluid to that of 

 a standard fluid may be accurately determined. 



It is not desirable to use a very strong solution of the substance to be 

 examined. The reason of this is the imperfect compensation which exists 

 between fluids possessed of opposite rotatory powers. It is generally as- 

 sumed that the ratio of the rotation produced in the planes of polariza- 

 tion of any two of the simple rays of which a white ray is composed is 

 the same, whatever be the substance causing the rotation. It follows, 

 indeed, from the law of Biot, that this is not accurately true, but it has 

 been generally supposed that the error is too small to be perceived. If 

 this were true, it would always be possible to assign to the lengths of 

 two columns of oppositely rotating fluids such a ratio, that the effect of 

 the one should be accurately compensated by the effect of the other. 



* Proceedings of the Royal Irish Academy, vol. vii., p. 349. 



