378 Mr Pocklington, On a method of increasing the 



where c, f are the ratios in which the amplitude is changed by 

 the refractions into and out of glass. Since b 2 + cf= 1, these can 

 be written 



where 6 2 = sin (i — r)/sin (i + r), 



and b 2 = tan (i — r)/tan (i + r). 



If i = 45° and fi=15 these parts of either beam are 

 •38 A cos <j> and '18 A sin <£, and to make these equal we must 

 have <f> = 65°. 



4. The main objection to be urged against this method of 

 obtaining two circularly polarised beams is that it requires that 

 the interval between the two inclined plates shall be very 

 accurately adjusted. Whether it is better to use this method, or 

 use the ordinary form of interferometer (with unsilvered inclined 

 plate) and allow for the want of exactitude of circularity in the 

 polarisation when calculating the result, can only be determined 

 by experiment. 



5. The phenomena in either case are rendered less brilliant 

 by the light reflected from the other face of the plate. This light 

 can be eliminated either by making the plates slightly wedge- 

 shaped, or by using a circular hole in a screen as a source of light 

 and forming a real image of it beyond the apparatus by means of 

 a lens. The light to be eliminated now passes through an image 

 distinct from that formed by the light which we want, and can be 

 cut off by a screen. 



[Note, Received 14 March 1902.] 



One of the most sensitive Polaristrobometers is Laurent's 

 form. In this the light in half the field has its plane of polarisa- 

 tion rotated through a certain small angle. On rotating the 

 analyser first one half and then, after a further small rotation, the 

 other half of the field becomes dark. The source of the great 

 sensitiveness is partly that the ratio of the two intensities varies 

 from to oo for a small rotation of the analyser, and partly that 

 this rotation can be accurately measured. 



The same principle can be used with the interferometer with- 

 out utilising circularly polarised light. The mirror should be 



