318 Professor Airy on a new Analyzer, 



light is therefore exactly of that kind which cannot be transmitted 

 by the analyzer, or the corresponding point of the rings is black. 

 This evidently will not apply strictly to quartz: but it will apply 

 with tolerable accuracy when the rays make a considerable angle 

 with the axis of the crystal. 



The properties of Fresnel's rhomb suggest at once a method of 

 constructing such an analyzer as we require. It is well known 

 that if circularly-polarized light is incident on Fresnel's rhomb, 

 it emerges plane- polarized, and the position of the plane of po- 

 larization at emergence makes an angle of + 45° or — 45° with the 

 plane of reflection according as the incident light was right- 

 handed or left-handed. Let the light emerging from the rhomb 

 be received on an unsilvered glass at the polarizing angle, whose 

 plane of reflection makes the angle + 45° with that of the rhomb. 

 Now it is plain that if the light incident on the rhomb was 

 right-handed, it becomes plane-polarized in the plane of reflection 

 of the glass, and therefore is wholly reflected : if it was left- 

 handed, it becomes plane-polarized in the plane perpendicular to 

 the plane of reflection of the glass, and therefore is wholly sup- 

 pressed. This combination therefore (a rhomb and an unsilvered 

 glass at + 45°) has the property of wholly transmitting right-handed 

 circular light and of wholly suppressing left-handed circular light : 

 and in the same way it would appear that the combination of 

 a rhomb and an unsilvered glass at — 45° has the property of 

 wholly suppressing right-handed circular light and wholly trans- 

 mitting left-handed circular light. Since all polarized light (and 

 therefore light in general) may be represented by two pencils of 

 opposite circularly-polarized light, it follows that our combination 

 will have the power of resolving all light into two such pencils 

 and suppressing one of them. 



