CANE SUGAR. 



Transition Tint Device. White light is the effect on the eye of 

 red, orange, yellow, green, blue, indigo, and violet rays ; these rays are, when 

 polarized, differently rotated in their passage through active bodies ; if, then, 

 we have two nicol prisms arranged as in the preceding paragraph, with their 

 longitudinal axes parallel, and rotate, as by the interposition between the two 

 nicols of an optically active material, any one of these components of white 

 light through 90, that component will be eliminated, and will not reach the 

 eye of the observer ; if the yellow rays are thus eliminated the rest combine 

 to form a peculiar pale rose or lilac tint termed the transition tint ; the 

 yellow rays are rotated 90 by a plate of active quartz 3- 75 mm. thick. 



This device, originated by Ilobiquet, and constructed by Soleil, consists, 

 then, in the interposition between the analyser and polarizer of a plate of 

 quartz 3 '75 mm. thick, one half of which is of dextro- and the other of levo- 

 rotatory quartz ; then if the nicols are parallel the field of vision (using 

 ordinary white light) is a uniform pale rose tint, and the pointer attached to 

 the analyser should indicate zero. Now, if the analyser be rotated but a 

 little, or if the plane of polarization be rotated by the interposition of an active 

 material, the proportion of red, orange, &c., rays in the two halves of the field 

 that reach the eye of the observer is different, on the one side red on the other 

 side blue rays predominating. This contrast between red and blue is very 

 pronounced, and measurement of a rotation is obtained as described before by 

 rotation of the analyser through an angle equal in magnitude, but opposite in 

 sign to that due to the material whose activity is being measured. 



This instrument then obtains its reference point by the elimination of the 

 yellow ray of white light, and measures the rotation of this ray. Specific 

 rotations obtained with this instrument are referred to as [a,-] ; (French jaune = 

 yellow). 



The Half Shadow Devices. In all these instruments the 

 reference point is a uniformly tinted field ; on introducing a very small 

 rotation one half of the field becomes darker in tint than the other, the change 

 being very sharp. With rotation far removed from that giving the reference 

 point a field of vision approximately uniformly tinted is also observed, and may 

 be distinguished from that connected with the reference point by there being 

 no sharp change on introducing a small rotation of the plane of polarization. 



The Jellet-Cornu Half Shadow Device. This half shadow 

 arrangement was originally devised by Jellet 3 in 1860, and since then has been 

 elaborated by many physicists. As made by Jellet, between the polarizing and 

 analysing nicols, and close to the former, was interposed a right prism of Ice- 

 land spar ; this prism was sawn through lengthwise, the opposite faces ground 

 down to equal angles, and the two halves cemented together in reversed 

 positions, so that they made an angle a with each other ; this angle is called 

 the half shadow angle. The Jellet prism was modified in 1870 by Cornu 4 



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