164 Dr. J. Kerr on Reflection of Polarized Light 



When the incidence is between grazing and principal, 90° 

 to 85°, and the two Mcols are initially at pnre extinction, any- 

 very small rotation of the first Mcol may be neutralized per- 

 fectly, or very nearly, in the polariscope, by a small rotation 

 of the second Mcol, provided the two rotations are both right- 

 handed or both left-handed, and properly related to each other 

 in magnitude ; but if the two rotations are one right-handed 

 and the other left-handed, the effect of one in the polariscope 

 is strengthened by the other ab initio. 



When the incidence is between principal and normal, 65° 

 to 30°, and the two Mcols are initially at pure extinction, any 

 very small rotation of the first Mcol may be similarly neutra- 

 lized by a small rotation of the second Mcol, provided the 

 rotations are one right-handed and the other left-handed ; but 

 if the rotations are both right-handed or both left-handed, the 

 effect of one is strengthened by the other ab initio. 



When the incidence is about 75°, and the two Mcols are 

 initially at pure extinction, no small rotation of the first Mcol 

 can be neutralized, or its effect in the polariscope sensibly 

 weakened, by a rotation of the second Mcol either way. 



It appears from these observations, that the arrangement 

 of the two Mcols for extinction is very sharply defined at or 

 very near the principal incidence only. At other incidences 

 there is a sensible though small range of angular magnitude, 



7T 



increasing as the angle of incidence approaches either -^ or 0, 



through which we can turn the first Mcol either way from pure 

 extinction, without losing the power to recover a pretty good 

 extinction by a proper displacement of the second Mcol. 



The next observation requires a compensating slip of glass. 

 That which I generally employ is a piece of good plate, \ of 

 an inch thick, 1 wide, and 7 or 8 long. It is held between the 

 mirror and the second Mcol, its plate-faces perpendicular to 

 the reflected ray, and its length at 45° to the plane of reflection. 

 If the slip has been properly chosen, and is in good condition, 

 it is quite inactive in the polariscope, except when purposely 

 strained by the observer's hands. The only strains applied 

 are compression along the length and tension along the 

 length. 



When the angle of incidence is about 75°, and the two 

 Mcols are initially at pure extinction, any very small rotation 

 of the first Mcol is neutralized perfectly or very nearly by the 

 compensating slip feebly strained : the right-handed rotation 

 is neutralized by compression right hand down, and, therefore, 

 also by tension left hand down ; and the left-handed rotation 

 is neutralized by tension right hand down, and, therefore, also 



