S2S Prof. C. V. Raman and Mr. G. L. Datta on 



appear. The structure (which is radial * and fibrous in 

 white light, and granular in monochromatic light) arises 

 from the irregular distribution of the diffracting particles or 

 heterogeneity of the film which dims the surface of the 

 mirror and is accordingly fundamental to the experiment. 

 Its effect may be minimized by increasing the aperture of 

 the observing telescope or diminishing the thickness of the 

 mirror, but considerations of convenience and the difficulty 

 in obtaining very thin mirrors with optically plane and 

 parallel faces limit the possibilities in either direction. 



The present authors have found two methods of experi- 

 menting by which it is possible to push the observations of 

 Quetelet's rings to angles of incidence right up to 90°. The 

 first method is to use a front-silvered glass plate (ordinary 

 \ in. mirror glass does very well), and to coat the surface of a 

 second glass plate with a scattering film and place it face to 

 face with the silvered surface on the first plate, thin slips of 

 paper or mica being placed at the corners between the two 

 plates to adjust the distance between them, In this way, 

 very satisfactory optical surfaces separated by any desired 

 space may be obtained. The authors have found that a light 

 coat of ammonium chloride on the surface of the plate 

 (deposited by volatilization) gives a highly homogeneous 

 and uniform scattering film of which the thickness can be 

 accurately controlled. This gives much more satisfactory 

 results than the film of milk which is usually recommended, 

 the granulation of the field being practically absent. 



The second method is to use a clear sheet of mica which 

 can be obtained readily and split to any desired thickness, 

 and to coat one of its surfaces with a thin film of ammonium 

 chloride by volatilization. The back of the mica may be 

 silvered if so desired, but this is not essential if a bright 

 source of light be available. As is well known, the surfaces 

 of a good sheet of mica are parallel to an extreme degree of 

 accuracy, and if the sheet be suitably held, it need not 

 appreciably deviate from planeness. 



By both methods, it is possible to obtain such small separ- 

 ations between the mirror and the scattering film that it is 

 entirely unnecessary to use an observing telescope, even at 

 very oblique incidences. Gorgeously-coloured rings are 

 seen when the arrangement is held close to the eye, and a 

 small source of light is viewed by reflexion from it. Using 



* With an obliquely-held plate, the streamers instead of running' out 

 quite radially from the reflected image of the source, curve unsvm- 

 metrically as shown in fig. 1. 



