294 PROCEEDINGS OF THE AMERICAN ACADEMY 



Investigations on Light and Heat, made and published wholly or in part wrra 

 Appropeiation from the Rumford Fund. 



XVII. 



A NEW FORM OF POLARIMETER. 



By Edward C. Pickering. 



Presented May 26, 1885. 



Two instruments have been proposed for measuring the relative 

 intensity of the components of a ray of polarized light. First, the 

 Arago polarimeter, in which a number of glass plates are placed in 

 front of a Savart polariscope, and may be inclined at any desired angle. 

 Bands are produced by the Savart plate, which are made to disappear 

 by inclining the plates so as to neutralize tlie polarization of the beam. 

 The inclination of the plates is a measure of the intensity of the polari- 

 zation. Secondly, a polarimeter was devised by the writer,* in which 

 the light is admitted through a rectangular aperture, two juxtaposed 

 images of which are formed by a double-image prism. If the light is 

 polarized, the two rectangles will be of miequal brightness. Interpos- 

 ing a Nicol prism, the two images may always be rendered equal by 

 rotating the Nicol. The intensity of the polarization is measured by 

 the cosine of the angle of rotation. The principal objection to the 

 Arao-o polarimeter is, that the law connecting the inclination of the 

 plates and the amount of polarization is extremely complex, and is best 

 determined experimentally in each case. This instrument is also ill 

 adapted to measure a ray which is almost perfectly polarized, but when 

 the polarization is slight, the well-known sensitiveness of the Savart 

 bands renders the accidental errors small. The other form of polar- 

 imeter is open to the objection that it is not very sensitive when the 

 polarization is slight. On the other hand, the amount of polarization is 

 directly connected with the angle of rotation of the prism by a simple 

 formula, and when a ray is strongly polarized it gives excellent results. 

 The advantages of both instruments appear to be combined in the 

 following modification ot the second instrument. The greater sensi- 



* Proc. Anier. Acad., IX. 1. 



