279 



The Eev. John H. Jellett read a paper — 



On a new Optical Sacchaeometee. (Plate XXII.) 



The author said that his attention had been directed to the possibility 

 of applying the new analyzing prism, the construction of which he had 

 described to the Academy some time since, to the construction of a sac- 

 charometer, capable of giving more accurate results than those obtainable 

 by means of the instrument of Soleil. Having described this latter in- 

 strument, he said that, as far as he could judge, both from his own ex- 

 periments and the report of others who had used it, the error to which 

 even an accurate observer would be liable in attempting to estimate the 

 strength of a saccharine solution, could not be reckoned as less than half 

 a grain per cubic inch for a single observation. Having stated what he 

 believed to be the cause of this want of accuracy, the author exhibited 

 and described the instrument which he had himself devised for the 

 same purpose. Of this instrument, the accompanying diagram (Fig. 1 ) 

 is a representation. 



aa is a short tube, containing two large lenses, serving to condense 

 the light of a lamp, which is placed as nearly as possible in the principal 

 focus of the lower lens, hb, cc, is a short tube, carrying at one extre- 

 mity a lens, cc, and at the other extremity a diaphragm, hh, pierced at 

 its centre by a very smaU hole, 0, which is situated in the principal 

 focus of the lens cc, and also, when the instrument is adjusted, in the 

 principal focus of the upper lens a. Ey this arrangement a beam of light 

 is obtained emerging from cc, sensibly parallel to the axis of the tubes. 

 This beam is polarized by being transmitted through a ]N^icol's prism, 

 contained in the tube dd. ee is a vessel, pierced at the lower end by a 

 circular hole, which is closed with plate glass. This vessel contains a 

 fluid, possessing a rotative power opposite to that of the fluid under ex- 

 amination. This latter fluid is contained in the tube ff, which rests on 

 the two upright pieces 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 instrument. The transverse piece, vv, is capable of sliding along 

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

 passing round a spindle with a matted head, attached to vv. By these 

 means a motion can be given to the tube j^/" 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, /, of the tube is in con- 

 tact with the piece of glass covering the lower aperture in the vessel ee. 

 It is plain, then, that the numbers read on the scale, which is graduated 

 so as to be read to 0 inch -001, will denote the length of the column of 

 fluid ^ F (Fig. 2) interposed between the bottom of the vessel and the 

 bottom of the tube, gg is an analyzing prism, constructed as before de- 

 scribed.^' hh is a lens, and I a diaphragm, with a small hole, at which 

 the eye of the observer is placed. The polarizing and analyzing 

 prisms are fixed in their places by small screws, a-, a-', each passing 



"Proceedings of the Royal Irish Academy," vol. vii., p. 348. 



