Mr. W. Baily on the Optical Properties of Starch. - 125 



In an oval grain of starch seen sideways the nucleus is 

 situated generally towards one end, and the arms of the cross 

 are curved, except when the length of the oval is parallel or 

 perpendicular to the plane of polarization, in which case two 

 arms form a straight line along the length of the oval, and the 

 other arms are curved. The same effect would be produced 

 by a crystalline disk constructed as follows : — Round a point 

 as nucleus draw a family of ovals, 

 each having the point towards one 

 end, and its greatest length in a cer- 

 tain direction, and let the axes of 

 elasticity in the disk be at each point 

 parallel and perpendicular to the curve 

 through that point. The form of the 

 cross for any position of the lower 

 Nicol will be given by lines drawn 

 through all the points at which the 

 curves are parallel or perpendicular 

 to the plane of polarization. 



Now in a round grain, or an oval grain seen endways, the 

 foreshortened parts of the coats give a series of circles, and 

 the optical effects are those of the circular disks considered 

 above ; and if the oval grain is seen sideways the foreshortened 

 parts of the coats give a series of ovals, and the optical effects 

 are those of the oval disk. Let us assume that the coats are 

 doubly refracting, and that one axis of elasticity at each point 

 is normal to the coat, and differs in value from the other Wo 

 which lie in the tangent plane to the coat. These two will 

 of course be of equal value in spherical grains ; but in other 

 cases they may differ somewhat from one another, but less 

 than each differs from the normal axis. In each coat, at points 

 where the coat is not foreshortened, the normal axis will be 

 more or less in the direction of the ray, and the effect of the 

 coat on the light will be little or none, since the tangential 

 axes are nearly, if not quite, equal ; but where the coat is 

 most foreshortened the normal axis will be at right angles to 

 the ray, and the effect of the coat on the light will be greatest. 

 Hence the effect of the whole grain on the light will be nearly 

 the same as that of a section taking in the nucleus and those 

 parts of the coats which are most foreshortened. But in such 

 a section the optical structure would be exactly that of the 

 disks we have been considering, and which we have seen to 

 produce the same effect on light that a grain of starch pro- 

 duces ; and therefore we may conclude that a grain of starch 

 really consists of a series of coats round a nucleus, each coat 

 being doubly refracting, and having one axis of elasticity 



