i84 SCIENCE PROGRESS 



In the case of absorption lines the phenomena are sHghtly 

 different. Both the gelatine effect and the developer effect 

 then cause a repulsion ; the turbidity effect causes an attraction, 

 and there is a further attraction due to a difference in the 

 sharpness of the outer and inner edges caused by turbidity 

 and halation. In general, the total effect is an attraction, 

 about one micron for normal exposures, but larger for over- 

 exposures. 



The investigations of Ross on the mutual action of adjacent 

 photographic images have resulted in the suggestion that the 

 displacements of star images on the recent eclipse plates, which 

 have been held to confirm Einstein's prediction of the deflection 

 of light by a gravitational field, may have been in reality a 

 photographic effect of a similar nature. Under the conditions 

 of the eclipse plates, Ross's results would not indicate any 

 distortion of the amount required ; but the matter has been 

 finally disposed of by a paper read by F. Slocum at the last 

 meeting of the American Astronomical Society. Two pairs 

 of photographs of the Pleiades were obtained, and one plate 

 of each pair was exposed to a circle of light, so that an image 

 31 mm. in diameter and somewhat denser than the corona was 

 printed on it. One pair of plates was developed with pyro, 

 the other with hydroquinone. Fourteen faint stars, sym- 

 metrically distributed around the central image, were measured 

 and the coefficient of distortion, which was assumed to vary 

 inversely with the radial distance from the disc, was determined. 

 The values in each case were small and of the order of their 

 probable errors, and were only a few per cent, of the coefficient 

 of distortion, which would be necessary to account on this 

 hypothesis for the measured displacements on the eclipse plates. 

 The photographic distortion in the case of the eclipse plates is 

 therefore negligible. 



The following is a selection from recent papers : 



Chazv, J., SurlaStabi]it6 avec la Loi du Cube des 'Dista^nces, Bull. A sir., Ser. II, 

 pt. i, 1, 151, 1901. Sur les Solutions isosceles du Probldme des trois 

 Corps, ibid., 1, 174, 1921. 



See, T. J. J., Researches on the Figure of the Earth, with definitive deter- 

 mination of the Oblateness and complete tables of the Corresponding 

 Terrestrial Spheroid, Ast. Nach., 213, Nos. 5103-4, 1921. 



Leckie, a. T., Analytical and Numerical Theory of the Motions of the Orbital 

 Planes of Jupiter's Satellites. Secular Terms. Leiden Obs. Annals, 

 12, pt. ii, 1919. 



KiENLE, H., Untersuchungen iiber Saalrefralrtion., Ast. Nach., 213, No. 51 11, 

 1921. 



Flint, A. S,, Meridian Observations for Stellar Parallax. Second Series. 

 Washburn Obs. Pubs., 13, pt. i, 1919. 



DuGAN, R, S., The Eclipsing Variable U Cephei, Princeton Univ. Obs. Contri- 

 butions, No, 5, 1920. 



