Mr Pocklington, On Michels on’s Interferometer. 
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On a method of increasing the sensitiveness of Miclielsons 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 § 5 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. 
