156 THE MICROSCOPE. 



Molecular Rotation. For the purpose of studying 

 the various interesting phenomena of molecular rotation, 

 a few necessary pieces of apparatus must be added to 

 the microscope. First, an ordinary iron three-armed 

 retort stand, to the lower arm of which must be 

 attached either a polarising prism or a bundle of glass 

 plates inclined at the polarising angle. In the upper 

 an analysing prism. The fluid to be examined should be 

 contained in a narrow glass tube about eight inches iii 

 height, and this must be attached to the middle arm. 

 If the prisms be crossed before inserting a fluid, 

 possessing rotatory power, the light passing through 

 the analyser will be coloured. If a solution of sugar 

 be employed, and the light which passes through the 

 second prism is seen to be red, but on rotating the ana- 

 lyser towards the right, the colour changes to yellow, 

 and passes through green to violet, it maybe concluded 

 that the rotation is right-handed. If, on the contrary, 

 the analyser requires to be turned towards the left 

 hand, we conclude that the polarisation is left-handed. 

 These phenomena are wholly distinct from those 

 accompanying the action of doubly refracting sub- 

 stances upon plane polarised light. It is not easy to 

 explain in a limited space the course to be followed in 

 ascertaining the amount of rotation produced by 

 different substances. Monochromatic light should be 

 used. If we are about to examine a sugar solution with 

 the prisms crossed, the index attached to the analyser 

 must first be made to point to zero. The sugar is 

 then introduced, when it will be necessary to rotate 

 the analyser 23 to the right, in order that the light 

 may be extinguished. This is the amount of rotation 

 for that particular fluid at a given density and that 

 height of column. As the arc varies with increase 

 or decrease of density and height of the fluid, it is 

 needful to reduce it to a unit of height and density. 

 The following formula is that given by Biot: P= 

 quantity of matter in a unit of solution ; d = sp. gr. ; 

 I = length of column ; a = arc of rotation ; m = mole- 

 cular rotation. Then m = -= -, 



l d 



