178 SCIENCE PROGRESS 



system otherwise complete. The conception of " conical order," 

 on which Dr. Robb founds his geometrical scheme, is already 

 familiar from his other treatises, and it is not necessary that 

 we should expound it here. But in the present small work 

 much that is new and suggestive appears, and it would seem 

 to be an essential part of the equipment of every geometer of 

 the future who is interested in the foundations of his subject. 

 The author promises us, in the near future, a further generalisa- 

 tion of his work, which he has already carried out to a certain 

 extent, though not published as yet. It was a common opinion 

 recently that relativity theory had, on the mathematical side, 

 gone as far as it could, and that its future lay with the physicist. 

 But works such as these by Prof. Eddington and Dr. Robb 

 clearly prove the contrary, as also does a long paper by Sir 

 Joseph Larmor, given to the International Congress of Mathe- 

 maticians at Strasburg last year, and now printed. We propose 

 to notice this in detail, together with others promised by their 

 authors in the near future, in subsequent articles. 



ASTRONOMY. By H. Spencer-Jones, M.A., B.Sc, Royal Observatory, 

 Greenwich. 



Photographic Action and Astronomical Measurement. — Photo- 

 graphic methods are to-day of great and increasing importance 

 in astronomical observation. For the determination of stellar 

 parallaxes and of the wave-lengths of lines in the spectra of 

 celestial bodies they have almost entirely displaced visual 

 methods, whilst for the determination of stellar magnitudes and 

 positions they possess many advantages and are largely used. 

 But in many problems precautions have to be taken to prevent 

 the introduction of systematic errors, which may arise from 

 the complicated nature of the laws of photographic action. 

 Thus, for example, doubling the exposure on a given source 

 does not result in a doubling of the photographic sensation. 

 For the determination of stellar magnitudes it is not, therefore, 

 permissible to compare the exposure-times required to produce 

 images of a certain density or size. Detailed investigation of 

 the mechanism of photographic action and of the resulting 

 phenomena is in consequence desirable. Investigation on these 

 lines is in course of progress at the Research Laboratory of the 

 Eastman Kodak Company, and several papers deahng with 

 various phenomena have been published recently by F. E. Ross 

 in the Astrophysical Journal. 



" Photographic Photometry and the Purkinje Effect " 

 {Astrophysical Journal, 52, 86, 1920) deals with the photo- 

 graphic analogue of the Purkinje Effect. It has long been 

 known that if two sources of different colour produce equal 

 visual sensations, then if the intensities of both sources are 



