Mr Pocklington, On Michelsoris Interferometer. 375 



On a method of increasing the sensitiveness of Michelsons Inter- 

 ferometer. By H. C. Pocklington, M.A., St John's College. 



[Received February 1902.] 



1. The object of this paper is to suggest a modification of the 

 interferometer, which should have a considerably greater sensitive- 

 ness. This is done by making the interfering beams consist of 

 circularly polarised light, a retardation of one beam then causing 

 a rotation of the plane of polarisation of the resultant. This 

 enables us to make use of the great sensitiveness of the apparatus 

 used for measuring the rotation of the plane of polarisation. The 

 general method is described in § 2, and a particular method of 

 arranging the apparatus is discussed in § 3. An objection to this 

 method is mentioned in § 4, with an alternative method whereby 

 it is avoided. In § o it is shown how to eliminate the noxious 

 light reflected from the inclined plate of the interferometer. 



2. Let a beam of plane polarised light be incident on the 

 inclined plate. This beam can be resolved into two, polarised 

 respectively in and perpendicular to the plane of incidence. Each of 

 these beams is divided into two parts at the inclined plate. One 

 pair of these parts is reflected down to a mirror and then back 

 through the inclined plate to the eye. The ratio of these parts 

 as they reach the eye is not in general unity, but can be made 

 unity by suitably choosing the plane of polarisation of the 

 incident light. Let now a plate of mica or other suitable sub- 

 stance, of such thickness that it produces a relative retardation of 

 an eighth of a wave, be placed between the inclined plate and the 

 mirror that we are considering, and let its principal lines be 

 parallel to and perpendicular to the line of intersection of the 

 plate and the mirror. Since the light passes twice through the 

 mica, a relative retardation of a quarter wave-length is produced, 

 and the light, after passing through the inclined plate, is circularly 

 polarised. The same can be done 1 in the case of the light that 

 passes through the inclined plate, is reflected at the other 

 mirror, and is finally reflected by the inclined plate to the eye. 

 If the beams are oppositely polarised they will combine to produce 



1 This cannot be done exactly unless the plane of polarisation of the inci- 

 dent light is slightly altered, and the resulting circularly polarised beam has not 

 the same intensity as that previously mentioned. See below. 



