420 Measuring Refractive Index and Dispersion of Glass. 



of the source of light formed by the objective C about 

 3 metres away from the observing telescope. Such an image 

 will, of course, always send light into the uncovered portion 

 of the object-glass of the observing telescope in whatever 

 position the edge F may happen to be placed. The intensity 

 of the light that goes to form a flare-spot image, however, 

 is not great and exerts no disturbing effect upon the readings. 

 It is well, however, to shield off the major portion of the 

 extended source of light from the apparatus and, instead of 

 a simple straight edge backed by a large source of light, to 

 use one of the straight edges in a slit about 1 mm, in width. 

 It will be found that this reduces the general illumination 

 to almost negligible proportions. If it be considered worth 

 while the intensity of the flare image can be reduced further 

 by tarnishing the glass-air surfaces of the lens C along the 

 lines pointed out by H. Dennis Taylor in a patent speci- 

 fication No. 29561 (1904), entitled "A method of increasing 

 the brilliancy of the images formed by lenses." A paper on 

 the same subject by F Kollmorgen in the Transactions of 

 the Illuminating Engineering Society, vol. xi. (1916), might 

 also be consulted. The outer zones of a thin positive lens 

 may be regarded as built up of a number of small prisms 



oriented with their refracting edges tanoentiallv to the 



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circular boundary of the lens, and no difficulty is therefore 

 to be expected in dealing with such a lens in accordance 

 with the method of this paper. Owing to the pressure of 

 more urgent work, however, it has not been found possible 

 further to develop the method with a view to ascertaining 

 its accuracy when applied to high-power negative lenses with 

 appreciable edge-thickness. It was thought, nevertheless, 

 that an account of what has already been accomplished 

 might prove of some service to others who might be 

 interested in determining all the optical constants of a given 

 system. 



The author desires, in conclusion, to place upon record 

 his indebtedness to Dr. Glazebrook, the Director of the 

 National Physical Laboratory, for the facilities placed at 

 his disposal for carrying out this investigation. 



