50 On Starch and Unannealed Glass under the Polariscope. 



This is done in figures 9 to 13, in which cr is about 15°. In 

 fig. 9, p = 45°; in fig. 10, p lies between 45° and a ; in fig. 11, 

 p = cr ; in fig. 12, p lies between cr and zero ; and in fig. 13, p 

 is zero. 



Suppose now the light to be passed through an analyzer 

 placed with its plane of polarization in the direction TT'. 

 "When p = 45°, the quadrants about UF will be dark, and 

 those about V Y / will be light. They will gradually shade 

 into one another, there being no black or full light. As p di- 

 minishes, UTJ 7 becomes darker until, when p = o", UtJ 7 is 

 black (see fig. 11). As p further diminishes, the black bar 

 opens out into a dark oblique cross, neither bar of which is 

 black ; and when p becomes zero, this cross becomes rectan- 

 gular and black. As p passes on to — o-, the cross becomes 

 oblique and not black, and closes up into a black bar along 

 VY 7 ; and when p becomes —45°, the quadrants about VV 

 are dark, and those about U U / light. When we have the 

 oblique cross, we can by a suitable turn of the analyzer make 

 either arm of the cross black. (See fig. 12.) 



If the analyzer is placed with its plane of polarization in the 

 direction S S', we get the same set of appearances, except that 

 we get light for dark and dark for light ; and in the case of 

 the single bar and rectangular cross, we get full light instead 

 of black. 



The appearances presented when a is variable may be well 

 seen in cylindrical disks of unannealed glass. I do not know 

 of any bodies which show very clearly the appearances pre- 

 sented when a is constant. Crystals of salicene show the 

 black cross remarkably well, and give indications of the single 

 black bar; but in this substance <r, though constant along 

 each radius, varies in passing from one radius to another, and 

 this completely hides the phenomena of the oblique cross. 

 However, in grains of tous-les-mois starch, phenomena closely 

 analogous to those above described as presented when a is 

 constant may be easily observed under a moderately high 

 power — the only difference in the phenomena being that, in 

 consequence of the grain of starch being generally an un- 

 symmetrical body, the lines are distorted, the black cross, 

 for instance, being neither rectangular nor rectilinear. See 

 " The Optical Properties of Starch," Phil. Mag. for August 

 1876. 



