i74 



NATURE 



[April 7. 1921 



to place over the object glass of the telescope a 

 coarse grating of parallel wires ; from the dimen- 

 sions of the grating the magnitude difference be- 

 tween the principal and first diffracted image can 

 be calculated. Thus one of the Greenwich grat- 

 ings, illustrated in Fig. 3, which has wires of 

 diameter 1-72 mm., and a total grating interval 

 of 70 mm., produces a magnitude difference of 

 2-66m. An enlargement of a portion of a photo- 

 graph obtained by this grating is shown in Fig. 4.2 

 It will be seen that the first diffracted images are 

 round, but that the second diffracted images are 

 elongated by dispersion, and not suitable for com- 

 parison. The use of the grating has the advantage 

 that all the information required can be obtained 

 from one exposure, the principal and diffracted 

 images corresponding to two series of images dif- 

 fering by a known magnitude. Any possibility of 



Fig. 3. — Diflfraction grating us::d in stellar pliotjmetry. 



error, which might otherwise be introduced owing 

 to a change in atmospheric conditions between 

 the exposures, is thus avoided. Other methods 

 which have been employed, principally at Harvard 

 and Mount Wilson, consist in using wire-gauze 

 screens, or rotating sectors, the reduction in 

 luminosity being measured by a photometer in the 

 laboratory in the first case, and calculated in the 

 second case, or in the reduction of the aperture 

 by circular diaphragms. Although the latter 

 method changes the diffraction pattern of the 

 images, no disturbing effects seem thereby to be 

 produced ; it is, however, objectionable in the case 

 of a refractor, as the light passes through dif- 

 ferent parts of the object glass in the two cases, 

 and the difference in absorption introduces errors. 

 Other methods have been employed, but less fre- 

 quently than those just referred to. It is not con- 

 venient to reduce the magnitude too much at one 

 step, as errors are liable to arise. A reduction of 



2 On account of the difficulty o reproducing satisfactorily faint star 

 mages, Figs. 4 and 5 are not actual reproductions of photographs, but are 

 drawn from the photographs. For this reason some of the images do not 

 app;ar as true discs. 



NO. 2684, VOL. 107] 



about ^ni is a practicable limit. If stars of a 

 wide range of brightness need to be compared, it 

 is preferable to make the comparison by two steps. 

 The diameters of the star images increase with 

 the length of exposure. The images are com- 

 pared with a scale obtained by giving exposures, 

 preferably with the same instrument, on a real or 

 artificial star, the length of the exposures being 

 so graduated that the difference in magnitude 

 between consecutive images is very nearly con- 

 stant. The sizes of the star images are compared 

 with those of the scale, interpolation to tenths 

 being made between the scale images. The com- 

 parison becomes difficult for very bright or very 

 faint stars, so that it is customary to measure 

 only those images which come within a certain 

 interval of the scale ; for the brighter and fainter 



- •• .- ■ . • 



Fk;. 4. — Portion of photograph obtained with diffraction grating. 



Stars, shorter and longer exposures respectively 

 must be given. Corrections have to be deter- 

 mined and applied for the distance of the star 

 image from the centre of the plate, and for atmo- 

 spheric absorption, the latter as in the case of 

 visual photometry. The former correction ari.ses 

 from the curvature of the field ; if focussed exactly 

 at the centre, the edges of the plate are not quite 

 in focus, so affecting the size of the images. It 

 is convenient to make the focus come some- 

 where between the centre and the edge of the 

 plate. 



In this way the magnitudes of the stars com- 

 prising the north polar sequence have been deter- 

 mined. There is a good accordance between the' 

 several determinations of the magnitudes in the 

 range iom-i5m, but in spite of the extensive 

 investigations which have been made, there 

 remain, systematic differences between the 

 magnitudes obtained for the brighter stars at 

 Mount Wilson and Harvard which exceed 0-251)1, 

 and this discordance illustrates how much more 



