928 



SUGAR 



by means of the polarising saccharometer proposed by Biot and improved by Soleil. 

 The following is a description of this beautiful instrument : Two tubular parts, T T', 

 and T" T"', Jigs. 1919 and 1920, constitute the principal part of the saccharometer. 

 The light enters n, through a Nicol's prism q (shown separately,/^. 1919, at o), and 

 passes first an achromatic polarising prism p (shown separately at p) and afterwards 

 through a plate of quartz of double rotation at p', which is also shown at Q. This plate 

 is composed, of two semi-discs cut perpendicularly to the axis of crystallisation ; but 

 though exactly of equal thickness and equal rotating power, the one turns the ray to 

 the right, while the other turns it to the left. At p', the ray passes a plate of quartz 

 of single rotation, and at 1 1', two wedges of quartz endued with the power of rotation, 

 but in a contrary direction to the preceding plate. These two wedges are again repre- 

 sented at A (fig. 1920), and are so made that by turning the milled head B, the sum of 

 their thicknesses can be increased or diminished at pleasure, while the amount of thick- 

 ness is shown by the ivory graduated scale e e', and vernier v v'. Finally, the ray 



traverses nn analysing prism a, and an eye-piece L. If the instrument is directed to 

 the light the observer will see a luminous disc, bisected by a central line (produced by 

 the junction of the two semi-discs of quartz) of exactly the same tint, but which tint 



