533 



handed and right-handed rotations may be equal to each other. If this 

 be so, we shall have 



mF r - nA r = nA b - mF b \ 



whence 



m _ A b + A r 

 n ~ F b + F r ' 



In order to measure exactly the quantities A r , A b , F b , F b , it was 

 necessary in the first place to obtain rays as nearly homogeneous as pos- 

 sible. This was easily effected in the case of the red ray by transmitting 

 a solar beam successively through blue and red glasses — this combina- 

 tion, as is well known, only permitting the extreme red ray to pass. It is 

 not possible to obtain so homogeneous a ray at the other extremity of the 

 spectrum ; but the condition is very approximately fulfilled by trans- 

 mitting the ray through a solution of sulphate of copper supersaturated 

 with ammonia. The light transmitted by a sufficient thickness of this 

 fluid, though not homogeneous, is almost entirely composed of rays si- 

 tuated at the violet end of the spectrum, the red, orange, and yellow rays 

 being nearly extinguished. 



The following were the values obtained for a column of each fluid 

 whose length was four inches : — 



A r = 6° 58', 

 A b = 33° 36', 

 F r = 19° 45', 

 F b = 61° 23', 



Hence we find 



When a mixture made nearly in this proportion was traversed succes- 

 sively by solar light which had passed through a combination of red and 

 blue glass, and by light which had passed through a suflicient thickness 

 of the ammoniacal solution of sulphate of copper, the effect was very 

 striking. With blue light, the left side of the spectrum was almost quite 

 black, while the right side was a bright violet-blue ; with red ligh t, 

 the right side of the spectrum is nearly black, and the left side a bright 

 red. This appearance indicates that the fluid, through which the light 

 has been transmitted, is left-handed for the red ray, and right-handed 

 for the blue and violet. The actual rotations are — 



For red light, .... - 1° 45', 

 For blue light, . . . . + 1° 56' ; 



the mixture containing 67 parts of American turpentine, and 33 of 

 French turpentine. The ratio in which the two species of oil of tur- 

 E. I. a, peoc. — VOL. IX. 4 A 



