792 Mr. F. Twyman on an Interferometer 



time being under test). Then the distortion of the lens for 

 that obliquity is given by 



tan(U + AU) _ 1 ^_ 0-0005818AU 

 tanU ~ sin2U 



the magnification in the neighbourhood of the axis being 

 taken as unity. The graduation gives the angular rotation 

 of each deflector, and AU can be found from the tables 

 supplied with each instrumeut. 



Visibility of the Bands. 



Valuable information relative to the bands of the prism 

 interferometer will be found in a paper by Ghiild*, There is 

 so far no published information relative to the visibility of 

 the bands given by these lens interferometers. The following 

 remarks may be found useful in this respect. If instead of a 

 small aperture in the diaphragm 12 (fig. 6) an extended light 

 source be used, then it will be found that, the instrument 

 having been adjusted with a good lens, and observation 

 being made with an eyepiece in the telescope, a series 

 of concentric rings will be observed (fig. 12). These are 

 due to the variations of relative retardation of the inter- 

 ferometer beams due to the rays in the lens beam traversing 

 a longer or shorter path according to their inclination. Of 

 the ring system so observed it must be noted that each point 

 of it is contributed to by every part of the camera lens. Such 

 a ring system is also in general observable in the case of a 

 camera-lens which has aberrations ; and it is in this case 

 also true that every part of the camera lens participates in the 

 formation of every point of the ring system observed in the 

 eyepiece. In fig. 12 are shown side by side a diagram 

 illustrating the ring system observed in the eyepiece, and 

 the interference bands for a particular photographic lens. 



Let us suppose the eye of the observer receives light firstly 

 only from a point b and secondly only from a point b\ and let 

 us direct attention to a point of the lens A. Then if, when 

 light reaches the eye through the point b, the point A on the 

 lens appears bright, the same point on the lens will appear 

 dark when the light reaches the eye through the point o . It 

 is thus obvious that, unless the light reaching the eye is 

 obtained from a region which is small compared with the 

 breadth of a single ring as seen in the eyepiece, the inter- 

 ference appearances seen on the camera lens will be faint. 

 Obviously, then, to secure good visibility the light which 

 reaches the eye of the observer should come from the centre 

 of this ring system. In the adjustments described above this 

 * Proc. Phys. Soc. xxxiii. pp. 32, 40 (1920). 



