On Starch and Unannealed Glass under the Polariscope. 39 



The phase-displacements a, a 1} a are accordingly small quan- 

 tities of the same order, which in the limiting case 



W Wx w 







2irnQ l 27rnQx 27raQ 

 become =0. 



But the results deduced for this special case are the expres- 

 sion of those obtained experimentally by M. Hermann. If it 



W Wi 



can be shown that in fact the quantities ~ — ~ and ~ 



were very small values in his experiments, then the complete 

 agreement of the results of experiment with the general law 

 of induction is demonstrated. By a closer consideration of 

 the dimensions, the number of turns, and the resistances of the 

 coils and telephones employed by him, we can perceive that 

 the quotients mentioned actually were small values. 



V. Starch and Unannealed Glass under the Polariscope. 

 By Walter Baily*. 



[Plates I.-IY.] 



THERE are many bodies, of which a spherical grain of 

 starch and a circular plate of unannealed glass may be 

 taken as specimens, having an optical structure symmetrical 

 about an axis through the body. The object of this paper is 

 to investigate the state of the light which emerges from such 

 a body, when monochromatic light in any state of polarization 

 is sent through the body in the direction of the axis. 



In fig. 1 (Plate I.) let S S' and TOY be drawn through R 

 perpendicular to one another, and let U TJ 7 and V Y' bisect 

 the angles between them. Suppose a quarter-undulation plate 

 to be fixed parallel to the paper, with its axes parallel to 8 S' 

 and T T 7 — and the light to be passed perpendicularly to the 

 paper through a Nicol's prism having its axis perpendicular 

 to the paper and its plane of polarization inclined at an angle 

 p to the line S S', then through the quarter-undulation plate, 

 and then through the body, which is also to be placed with its 

 axis perpendicular to the paper. 



Let the paper represent a section of the light after it has 

 emerged from the body. Take any point P and draw round 

 it an ellipse representing the polarization of the light at P. 

 The state of the light will be completely determined if we 

 know the angle (a) which the axis major of this ellipse makes 

 with II P, the angle (/3) which a line joining the extremities 



* Bead before the Physical Society, June 20, 1878. 



