364 SCIENCE PROGRESS 



cathode was cadmium, and the gases were helium, neon, argon, 

 and xenon. By suitable arrangements it was possible to observe 

 the lines of cadmium along with the gas lines excited by the 

 discharge even at comparatively high pressures and low current 

 densities. It is difficult to account for the appearance of the 

 metallic lines. They can appear with very feeble currents and 

 the electrode and tube quite cold. The spectrum may be due 

 to small particles of the metal at a high temperature torn off 

 by the violent impact of positively charged gas ions, but this is 

 hard to reconcile with the behaviour of the lines in different 

 gases and under different pressures, as for instance the fact 

 that not all the spark spectrum of the metal is seen at once, 

 but only certain lines depending on the nature and pressure of 

 the gas in the tube. 



The Physical Review for June contains a paper by Wick 

 and McDowell on the luminescence of the uranyl salts under 

 cathode-ray excitation. The October number of the same 

 journal contains a paper by T. K. Chinmayanandam on the 

 diffraction of light by an obliquely held cylinder, with an 

 account of experiments made to test certain theoretical results. 



The Phil. Mag. for November 191 8, and the Proc. Phys. Soc. 

 for December 191 7, Feb. and June 191 8, contain papers by 

 T. Smith on such topics as tracing rays through optical systems, 

 use of approximate methods in obtaining data for the con- 

 struction of telescope objectives, and on multiple thin objec- 

 tives. Other optical papers in the thirtieth volume of the 

 Proc. Phys. Soc. are : " An exhibition of the uses of certain 

 methods of classification in Optics," by T. H. Blakesley. " The 

 primary monochromatic aberrations of a centred optical 

 system," by S. D. Chalmers. " Note on the Pulfrich Refracto- 

 meter," by J. Guild. " The accuracy attainable with critical 

 angle refractometers," by F. Simeon. 



The August number of the Proc. Roy. Soc. contains a very 

 interesting account of a statistical research which has been 

 carried out in the colour-perception of about eighty students 

 of the University of Glasgow, by Dr. Houstoun of the staff of 

 that University. The object was to determine if normal colour- 

 vision passes gradually into colour-blindness, or if the colour- 

 blind form a well-defined class by themselves. Thus each 

 person tested was assigned a " mark " which indicated the 

 extent of his colour-vision, the " mark " being 1 for total 



