172 Dr. J. Kerr on Reflection of Polarized Light 



The reader would have a false view of these two experi- 

 ments, if he thought there was any thing uncertain about the 

 phenomena, any mere guess-work in the observations. On the 

 contrary, the effects are perfectly regular, almost invariably 

 very sensible, and sometimes beautifully distinct. In one or 

 two trials which I conducted in favourable circumstances along 

 with several friends, I found that the movements of the band, 

 whether produced by currents or by displacements of the 

 second Mcol, were sufficient to make a strong, correct, and 

 immediate impression upon an eye quite uneducated, and 

 which had been merely directed to the right point. 



I think that any discussion of the optical phenomena here 

 utilized would be quite irrelevant. 



17. In the next two experiments, I apply the compensating 

 slip of glass already described (art. 5). It is introduced into the 

 path of the reflected light, between the mirror and the second 

 Nicol, its length at 45° to the plane of reflection, and its plate- 

 faces perpendicular to the ray. Of the four methods of strain- 

 ing the glass, I generally choose compression right hand down. 

 To apply the action, I hold the slip at the lower end, between 

 finger and thumb of the right hand, taking care to have the 

 piece properly directed, and exert a small pressure downwards 

 by the fore-finger of the left hand laid along the upper end of 

 the slip. A very feeble pressure is sufficient to give a 

 perceptible restoration of the reflected image from pure 

 extinction. 



18. Seventh Experiment. — The plane of polarization of the 

 light incident on the mirror is parallel to the plane of incidence ; 

 and the two Mcols are kept at pure extinction. 



When the light is restored very faintly by the compressed 

 glass, it is not sensibly strengthened or weakened by either 

 magnetization of the mirror ; and this holds true for all inci- 

 dences between 85° and 30°. Sometimes, indeed, when 

 working in this way, I obtained sure though very faint changes 

 of intensity in the polariscope — an increase by one current, and 

 a decrease by the other ; but in several cases where these effects 

 were closely examined, they were found to be caused by imper- 

 fection of initial extinction, and particularly by slight mis- 

 placements of the second Nicol. Upon the whole, if there 

 was any regular effect here, it was too faint to be certainly 

 characterized. 



19. Eighth Experiment. — The plane of polarization of the 

 light incident on the mirror is perpendicular to the plane of 

 incidence ; and the initial extinction is made as pure as possible, 

 particular attention being given to the placing of the first 

 Nicol. Other things are as in the seventh experiment. 



