358 Prof. Jellett on a new Optical Saccharometer. 



circular hole, which is closed with plate glass. This vessel con- 

 tains a fluid possessing a rotative power opposite to that of the 

 fluid under examination. This latter fluid is contained in the 

 tube ff 9 which rests on the two upright pieces y y. These pieces 

 are attached to the transverse piece vv, which carries a vernier 

 whose divisions correspond to those of the scale ss, which is 

 attached to the bar zz, which carries all the parts of the instru- 

 ment. The transverse piece vv is capable of sliding along z z, 

 this motion being produced by a chain attached at both ends to 

 z z, passing round a spindle with a milled head, attached to v v. 

 By these means a motion can be given to the tube // parallel to 

 its own axis; and by a very simple arrangement the zero of the 

 vernier is made to coincide with the zero of the scale when the 

 extremity, f, of the tube is in contact with the piece of glass 

 covering the lower aperture in the vessel e e. It is plain, then, 

 that the numbers read on the scale, which is graduated so as to 

 be read to O001 inch, will denote the length of the column of 

 fluid EF (fig. 2) inter- 

 posed between the bottom 

 of the vessel and the bot- 

 tom of the tube, g g is an 

 analyzing prism, construc- 

 ted as before described*. 

 h h is a lens, and / a 

 diaphragm with a small 

 hole, at which the eye of 

 the observer is placed. 

 The polarizing and ana- 

 lyzing prisms are fixed in 

 their places by small 

 screws, cr, a, each passing through a transverse slit in the outer 

 tube, so that when partly unscrewed they allow the prisms 

 to turn through a small angle round the axis of the tube. In 

 using the instrument, the polarizing prism may be set in any 

 position, the analyzing prism being then carefully adjusted so 

 that the tints in the two halves of the circular spectrum f may, 

 when there is 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 e e may be French 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 //is then filled with a solution of sugar whose strength is 

 accurately known. The tube is now replaced in the upright 



* Proceeding's of the Roval Irish Academy, vol. vii. p. 348. 

 t Ibid. p. 349. 



