N 2 - v 2 



N 2 



18 



-- , where \u + ' denotes the 



A" 



400 MR. J. C. MAXWELL GARNETT ON 



tube containing some emanation. Within two days an unmistakable ruby tint 

 appeared in the glass.* 



It seems probable that the violet coloration of soda glass bulbs used in the 

 production of Rontgen rays may be due to the excretion of metal caused by the 8 

 rays from the cathode. 



The observations of ELSTER and GEITEL, ' Wied. Ann.,' 59, p. 487, 1896, quoted by 

 J. J. THOMSON, ' Conduction of Electricity through Gases,' p. 496, that salts of the 

 alkali metals coloured by exposure to cathode rays exhibit photo-electric effects, 

 suggestive of the presence of traces of the free metal, support this view as to the 

 cause of the coloration of metal glasses exposed to the radiation from radium. 



From equation (6), as modified for the case when the metal sphere is surrounded 

 by glass of refractive index i>, it appears that the amplitude at any point of the light 



emitted from the sphere is proportional to 



modulus, + v'n- + '''"' Using a and 8 as defined in equation (13), we have 



W2 2 2 



= a~ + 4/3 2 , where a and 8 are to be found from the table on p. 396, where 

 N 2 -4- 2v 



v = L'56. Thus at any point the intensity of light emitted by a sphere of radius a 

 is proportional to (a 2 + 4/3 2 )/X 4 = I, say. Measuring X in millim./lOOO, the Table 1. 

 gives the following values of T I- 

 Silver. Copper. Gold. 



Yellow (X = -589) I, = 27'95 62-11 70-88 



Red (\=-630)I., = 38-81 2175 3479. 



From these values of I it appears that when white light falls on a small sphere the 

 light emitted is, for 



Silver, more red than yellow, I ; > I y , 

 Copper ,. yellow ,, red, I y > I,, 

 Gold L > I s . 



The presumption is that for the two latter the light may be more green than 

 yellow. 



In the table given by SIEDENTOPF and ZSIGMONDY (loc. cit.), of which a copy is 

 given (Table II., p. 397), it is seen that of the five glasses Cc, E, F, G, H, whose 

 particles are small compared with a wave-length of light in the glass, the four glasses 

 Cc, F, G, H contain particles which send out a green cone of light, and the glass E 

 contains some particles which send out green and some which send out broim. 



Thus far -we have confined attention to glasses for which the condition of having 



* [Note added \\tli May, 1904. Sir WILLIAM RAMSAY has lately exposed some clear silver glass and 

 some soda glass at the same time to the emanation from radium. After a fortnight's exposure the silver 

 glass had turned a faint yellow and the soda glass a deep blue-violet.] 



