712i Prof. R. W. Wood : Some Experiments on 



oxygen destroys the luminosity of the emission. This fact 

 appears to be of the greatest importance in connexion with 

 the origin of these bands. If I remember rightly, the intro- 

 duction of chlorine gas into a sodium flame destroys its 

 emission of the D lines, and there may be some analogy 

 between the two phenomena. I intend sometime to photo- 

 graph the spark directly utilizing the principle of the spectro- 

 heliograph. An image of the spark obtained with mono- 

 chromatic light of the wave-length of the water-band may 

 tell us something about the origin of the emission. In the 

 meantime I hope that some study of the phenomenon will 

 be made by others, as it appears to be of considerable im- 

 portance in connexion with the origin of radiation. 



It seems quite likely that the " Entladungsstrahlen " may 

 be identical with the emission, for they are absorbed by 

 oxygen. One great difficulty in the investigation is the 

 apparent impossibility of separating the emission from the 

 ultra-violet and visible light which goes out with it. 



LXXVTII. Some Experiments on Refraction by 

 non-homogeneous Media. By H. W. Wood *. 



[Plate XIV. fig. 10.] 



ri^HAT the apparent diameter of a body surrounded by a 

 JL refracting atmosphere is slightly larger than its true 

 diameter is well known. An extreme case is the mercury 

 thread of a thermometer. At the other extreme we have the 

 earth as seen from the moon. 



This magnification by a non-homogeneous atmosphere, in 

 which there is no sharply defined refracting surface (as in 

 the case of the earth's atmosphere) can be very nicely shown 

 in the following way : — 



Make a small rectangular glass tank by cementing five 

 squares of glass together with sealing-wax. Fill it with 

 melted gelatine and support an empty test-tube in the fluid 

 with a clamp stand. The bottom of the test-tube should be 

 within half a centimetre of the bottom. After the jelly has 

 solidified, pour hot water into the test-tube, and immediately 

 withdraw it. It will leave a cylindrical hole in the jelly, 

 with a hemispherical bottom. Now pour a mixture of 

 glycerine and powdered chalk into the cavity until it is half 

 lull. Fill the remainder with water to which a few drops 

 of milk have been added. The glycerine will gradually 



* Communicated by the Author. 



