Jellett — 
On Optical Saccharometry . 
475 
XXX VIII. — A FURTHER Communication on Optical Saccharometrt, 
WITH special reference TO THE SUGAR BeETS GROWN IN IRELAND IN 
THE TEAR 1872. By THE PRESIDENT. 
[Read to the Academy, 24th February, 1872.] 
The Author took occasion in the first place to describe to the Academy 
an improvement which he had effected in the construction of the 
Saccharometer. According to the original construction of this instru- 
ment, the tube containing the fluid under examination is made to 
plunge into the compensating fluid, the length of the column of this 
latter being determined by the position of the tube. As it is not 
always possible to preserve a perfect identity between the temperature 
of the compensating fluid and that of the surrounding air, the tem- 
peratures of the immersed and unimmersed portions of the tube are 
often slightly different. This unequal heating, which is necessarily 
communicated to the fluid contained in the tube, is found to destroy 
the perfect polarization of the light, which is absolutely essential to the 
accuracy of the experiment. 
According to the improved construction, the tube containing the 
fluid under examination remains fixed, and the length of the column 
of compensating fluid is regulated by the immersion of an empty tube, 
closed at the lower end with glass, and so placed as to be in direction 
with the first tube. The unequal heating of the fluid under examina- 
tion is thus avoided. 
Another advantage which may be obtained from this construction 
is, that it renders possible an arrangement by which comparative ob- 
servation of two fluids, an object of great importance in all such ex- 
periments, is greatly facilitated. Instead of a single pair of rests for 
the tube containing the assay, the instrument is famished with two 
pairs capable of holding two tubes in a position of accurate parallelism. 
These rests are set in a sliding plate, thus enabling the observer by a 
lateral motion to bring either into the field of view, and thus to make 
comparative observations in very rapid succession. 
Thus in the present experiment, one of these tubes is filled with 
the best syrup, and the other with a standard solution of cane sugar. 
The comparative strength of these two can be obtained with great 
accuracy. 
The author observed that in the examination of beet syrup, at least 
according to the process which he had employed, it is for all practical 
purposes sufficient to make this one comparative experiment, calcula- 
ting the quantity of sucrose in the syrup on the hypothesis that the 
rotation of the plane of polarization is due to this sugar alone. 
The approximate truth of this supposition depends upon the facts 
that the grape and inverted sugars are present in very small quantities, 
W. I, A. PROC. — VOL. I., SER. II., SCIENCE. 3 Q 
