L24 Mr. W. Baily o/i the Optical Properties of Starch. 



Under the polariscope, with the Nicols at right angles, the 

 grain appears to be marked with a black cross, whose inter- 

 section always coincides with the nucleus, whose arms reach 

 the circumference, and whose form depends on the kind of 

 starch, as well as on the position of the grain ; the spaces 

 between the arms are illuminated with less than full light, the 

 brightness increasing with the distance from the cross. If 

 the Nicols are made parallel the cross becomes white, and the 

 spaces between the arms become dark but not black, the dark- 

 ness increasing with the distance from the cross. Hence it is 

 evident that the light which emerges from the starch at points 

 on the cross is plane-polarized in the plane of polarization of 

 the lower Nicol, and that the light from points in the spaces is 

 elliptically polarized, the polarization being most nearly cir- 

 cular at points most distant from the cross, where the turning 

 of the upper Nicol was found to occasion the least change in 

 the illumination. In round grains, such as young grains of 

 potato-starch, or in oval grains seen endways, the cross is 

 rectilinear and rectangular, the arms being parallel and per- 

 pendicular to the plane of polarization. The appearance re- 

 sembles closely that given under the polariscope by the circular 

 disks formed in crystallized hippuric acid and salicene. In 

 hippuric acid the pattern is in black and white ; but in salicene, 

 though the cross is black or white, the remainder is brilliantly 

 coloured. In starch the pattern is in black and white generally, 

 but the largest grains show a trace of colour. 



Now the optical properties of these disks can be completely 

 explained by supposing that the substance is doubly refracting, 

 with two axes of elasticity at each point in the plane of the 

 disk, one of them being directed towards the centre of the 

 disk. The arrangement of the axes of elasticity in the disk is 

 therefore given by a family of lines radiating from the centre, 

 and a family of circles described about it. Plane-polarized 

 lio-ht passing through such a disk will emerge without change 

 along the diameters which lie parallel and perpendicular to 

 the plane of polarization ; for the vibration will coincide with 

 one of the axes of elasticity at every point on these diameters : 

 but at other points the light will be resolved into a vibration 

 parallel to each axis ; and as these two vibrations will be un- 

 equally retarded, the light will become elliptically polarized ; 

 and the change will be greatest in middle of the quadrants, 

 where the axes are equally inclined to the plane of polariza- 

 tion. The absence of colour in hippuric acid is due to the 

 difference of retardation being proportional to the wave-length, 

 whereby light of all kinds emerges similarly polarized : but in 

 salicene the proportion between the retardation and the wave- 

 length must be different for different kinds of light. 



