590 Dr. A. 0. Rankine and Dr. L. Silberstein 



on 



therefore proved that no ellipticity amounting to about 

 k = 0*05 comes into existence in a range of L = 40 metres. 



In order to carry the measurements further, it was necessary 

 to adopt a more sensitive method of detection, namely, one 

 based upon observations of change of light intensity. Two 

 modes of procedure were considered. Either one could: 

 polarize the light at 45° to the vertical, and examine the 

 extent of extinction obtainable by an analysing nicol, first, 

 close up to the polarizer, and then at increasing distances ;.. 

 or, fixing the polarizer and analyser at a definite considerable 

 distance apart, one could observe any possible effect on the 

 completeness of extinction caused by altering the orientation 

 of original polarization from 0° * to 45°. For several reasons 

 the second alternative seemed preferable. Of course, if it 

 were possible to construct a rigid framework upon which 

 polarizer and analyser could be separated to a great distance 

 from one another without altering at all their relative orien- 

 tation, the first alternative might be used. Such an instal- 

 lation would, however, be extremely costly, and, even so, it 

 is unlikely that it would give results superior to those 

 described in what follows. 



The arrangement finally adopted was this. The source of 

 light, a 100 candle-power pointolite lamp, was placed at the 

 focus of a lens of 50 cm. focal length which cbllimated the 

 light. The beam then proceeded through a large nicol which 

 served as polarizer. The edges of this nicol were about two 

 nches long, but only the central portion of it was used, a 

 stop of J inch aperture being interposed between the nicol 

 and the collimating lens. A small nicol, placed in the beam 

 of light 40 metres away, was used as analyser at the receiving- 

 station. Its scale was graduated in degrees, and read to 0°'l. 

 For the sake of greater accuracy, the levelling screw on the 

 stand of the instrument was calibrated so as to enable us to 

 measure much smaller rotations — down to o, 003. 



It soon became evident that in order to get satisfactory 

 extinction when the nicols were nominally crossed, lenses 

 and all glass must be dispensed with between the two nicols. 

 Even then there survived a small amount of light, which was 

 primarily due to scattering by dust particles at the interface 

 of the two halves of the polarizer. It was, however, very 

 feeble. At 40 metres distance, viewed through the analyser, 

 it needed careful watching, and often disappeared owing to 

 the wandering of the eye. Experiments were now carried 

 out after cessation of daylight in a long corridor in the-' 



* It will he obvious that even if c v -=£z Ch there will be no ellipticity 

 produced provided the light vector at the source is either horizontal or 

 vertical, aud a maximum effect if it is inclined at 45°. 



