178 Methods for detecting small Optical Retardations. 



become zero when f=0 or f =<r, the four fractions them- 

 selves always remain finite. The line of transition between 

 the two halves of the field is not so marked as when there 

 was an actual discontinuity in the retardation itself. 



In connexion with these calculations I have made for my 

 own satisfaction a few observations, mainly to examine the 

 enhanced brightness at the edges of the field of view. The 

 luminous border is shown in Draper's drawing, and is 

 described by Topler as due to diffraction. The slit and 

 focussing lens were those of an ordinary spectroscope, the 

 slit being drawn back from the " collimating " lens. The 

 telescope was from the same instrument, now mounted inde- 

 pendently at a distance so as to receive an image of the slit 

 and itself focussed upon the first lens. The rectangular 

 aperture at the first lens was originally cut out, of the black 

 card. The principal dimensions have already been given. 

 A flat paraffin-flame afforded sufficient illumination. The 

 screens used in front of the telescope were razor-blades 

 (Grillettes), and were adjusted in position with the aid of 

 an eyepiece, the telescope being temporarily removed. 

 It is not pretended that the arrangements used corresponded 

 fully to the suppositions of theory. 



The brightness of the vertical edge of the field of view is 

 very conspicuous when the light is partly cut off by the 

 advancing screen. A question may arise as to how much 

 of it may be due to light ordinarily reflected at the edges of 

 the first aperture. With the aperture cut in cardboard, 

 I think this part was appreciable, but the substitution of a 

 razor-edge at the first aperture made no important difference. 

 The strongly illuminated border must often have been seen 

 in repetitions of Foucault's experiment, but I am not aware 

 that it has been explained. 



To examine the sadden transition from one uniform retar- 

 dation to another, I used a piece of plate glass which had 

 been etched in alternate strips with hydrofluoric acid to a 

 depth of about JX*. When this was set up in front of the 

 first aperture with strips vertical, the division-lines shone 

 out brightly, when the intervening areas were uniformly 

 dark or nearly so. No marked difference was seen between 

 the alternate division-lines corresponding to opposite signs 

 of p. Perhaps this could hardly be expected. The whole 

 relative retardation, reckoned as a distance, is J\ ? and is 

 thus intermediate between the values specified in Table IV. 

 It would be of interest to make a similar experiment with a 

 shallower etching. 

 Terling- Place, Witham. 



Jan. 5, 1917. 

 * Compare ' Nature/ vol. lxiv. p. 385 (1901) ; t Scientific Papers ' 

 vol. iv. pp. 546, 547. 



