ASTRONOMY 185 



Sanford, R. F., The Orbits of seven Spectroscopic Binaries, Astroph. Journ., 



53, 201, 1921. 

 Merrill, P. W., Observations of the Nebular Lines in the Spectrum of the 



Long-period Variable Star R Aquarii, Astroph. Journ., 53, 375. 1921. 



Characteristic Behaviour of the Bright Lines in Stellar Spectra of Class 



Ma., ibid., 185, 1921. 

 Wright, W. H., The Spectra of Temporary Stars, pt. i, A Preliminary- 

 Account of the Spectrum of Nova Ophiuchi (1919)- l^ick Obs. Pub., 



14, pt. i, 1921. 

 St. John, C. E., and Nicholson, S. B., On Systematic Displacements of 



Lines in Spectra of Venus, Astroph. Journ., 53, 380, 1921, 

 Brown, E. W., On the Passage of a Star through a Nebula, Astroph. Journ., 



53, 169, 192 1. 

 Milne, E. A., Radiative Equilibrium in the Outer Layers of a Star : the 



Temperature Distribution and the Law of Darkening, M.N., R.A .S., 81, 



361, 192 1. Radiative Equilibrium and Spectral Distribution, ibid., 81, 



375, 1921. 

 Veronnet, a.. Constitution physique du soleil et des dtoiles ^tudiee au 



moyen de la formule des gaz reels. Bull. Astr., Ser. H, pt. i, 1, 83, 1921. 

 MicHELSON, A. A., and Pease, F. G., Measurement of the Diameter of a 



Orionis with the Interperometer, Astroph. Journ., 53, 249, 1921. 

 Kapteyn, J. C, and Van Rhijn, P. J., The Number of Stars between Definite 



Limits of Proper Motion, Visual Magnitude, and Galactic Latitude for 



each Spectral Class, together with some other Investigations, Groningen 



Pub., No. 30, 192 1. 

 Van Rhijn, P. J., On the Brightness of the Sky at Night and the Total 



Amount of Star-light, Groningen Pub., No. 31, 1921. 

 Kruger,F., Indexkatalog nebst erganzungen zu der Sternfarbenverzeichnissen 



von Hagen, Kriiger und Osthoff, Pub. Specola Astronomica Vaticana, 



No. 9, 1917- 

 Slipher, E. C, Photographing the Planets, with especial Reference to Mars, 

 Pub. Ast. Soc. Pac, 33. 127, 1921. 



PHYSICS. By James Rice, M.A., University, Liverpool. 



WeyVs Generalisation of the Einstein Theory. — In a former 

 article in Science Progress (vol. xiv, p. 557), the writer 

 endeavoured to give a brief and elementary exposition of 

 Einstein's General Relativity Theory. It was emphasised that 

 for the mathematical treatment the introduction of the concept 

 of the " separation " between " two events " was of prime im- 

 portance. In his earlier or " restricted " theory, Einstein had 

 adopted ^/{hx^ - Bx^ - Bx^"" - Bx^^) as the value of the separa- 

 tion between two events whose co-ordinates in a definite frame 

 of reference are x^, x^, x^, x^, and Xi + Bx^, etc., respectively. 

 It is implied that the space co-ordinates x^, x^, x^ are Cartesian. 

 The justification for the use of this measure lies in the fact 

 that if the co-ordinate system be altered to another which 

 is in uniform relati ve motion to the first , then \/{^^i~ 

 Bx^^ - Bx,"" - Bx,') = ViBx\^ - Bx\^ - B:xf,^ - Bx^') of the Lorentz 

 transformation equations are employed. That is, the expression 

 in question is invariant. This invariance, in fact, takes the place 

 of the assumed individual invariance of V{Bxi + Bx^^ 4- Bx^) 



