162 Lord Rayleigh on Methods for detecting small Optical 



under difficult conditions interference-bands may be dis- 

 played in which a local departure from ideal straightness 

 amounting to ^o °f the band period can be detected on simple 

 inspection. I may instance some recent observations in 

 which the rays passing a fine vertical slit backed by a 

 common paraffin-flame fell upon the object-glass of a 3-inch 

 telescope placed some 20 feet away at the further end of a 

 dark room. No collimator was needed. The object-glass 

 was provided with a cardboard cap, pierced by two vertical 

 slits, each fa inch wide, and so placed that the distance 

 between the inner edges was -j 8 inch. The parallelism of 

 the three slits could be tested with a plumb-line. To observe 

 the bands formed at the focus of the object-glass, a high 

 magnifying-power is required. This was afforded by a small 

 cylinder lens, acting as sole eyepiece, Avhose axis is best 

 adjusted by trial to the required parallelism with the slits. 

 Fairly good results were obtained with a glass tube of 

 external diameter equal to about 3 mm., charged with water 

 or preferably nitro-benzol. Latterly, I have used with 

 advantage a solid cylinder lens of about the same diameter 

 kindly placed at my disposal by Messrs. Hilger. With this 

 arrangement a wire stretched horizontally across the object- 

 glass in front of the slits is seen in fair focus. When the 

 adjustment is good, the bands are wide and the blacknesses 

 well developed, so that a local retardation of ^o^ or ^ ess * s 

 evident if suitably presented. The bands are much disturbed 

 by heated air rising from the hand held below the path of 

 the light. 



The necessity for a high magnifying-power is connected 

 with the rather wide separation of the interfering pencils 

 as they fall upon the object-glass. The conditions are most 

 favourable for the observation of very small retardations 

 when the interfering pencils travel along precisely the same 

 path, as may happen in the interference of polarized light, 

 whether the polarization be rectilinear, as in ordinary double 

 refraction, or circular, as along the axis of quartz. In 

 some experiments directed to test whether " motion through 

 the aether causes double refraction " *, it appeared that a 

 relative retardation of the two polarized components could be 

 detected when it amounted to only X/12000, and, if I remember 

 rightly, Brace was able to achieve a still higher sensibility. 

 The sensibility would increase with the intensity of the light 

 employed and with the transparency of the optical parts 

 (nicols, &c), and it can scarcely be said that there is any 

 theoretical limit. 



* Phil. Mag. vol. iv. p. 678 (1902) ; ' Scientific Papers,' vol. v. p. 66. 



