104 



Lord Rayleigh— Spectroscopic Notes 



advantage that the tubes through part of their length should 

 make a tolerably close fit. But the most important precaution 

 appears to be the use of very short sparks and a reduction 

 of the battery to two cells. When it is desired to observe 

 the spectrum, a small jar must be connected in the usual 

 way. 



The spectroscope employed had two prisms, and the sparks 

 were focussed upon a somewhat wide slit by a 2-inch lens. 

 A low-power eyepiece was favourable. 



The group of lines in the argon spectrum first observed by 

 Schuster* (fig. 2) is easily seen. Owing to the warmth 



43 44 45 



Fig. 2. 

 46 47 48 



J L_l 



49 5000 



M 



Vffi 



'071 



Med 



fync 



(fuydroge 



71 



H/ F 



F is very diffuse, sometimes nearly filling up the interval 

 between 4879 and 4847. On one occasion when the original 

 air taken was only 5 c.c, the group was almost as distinct 

 as with pure argon. The residual gas, measured cold, was 

 probably no more than '1 c.c. This was rather an extreme 

 case, and it would not have been possible to renew the sparking 

 without an addition of oxygen, to be afterwards removed by 

 careful additions of hydrogen. But the argon spectrum 

 shows fairly well even when the gas is diluted with two or 

 three times its volume of oxygen. 



I have described this experiment at some length because 

 I think that it would make a good exercise for students, 

 requiring no special apparatus but what they should be able 

 to construct for themselves. Although, as I have said, 5 c.c. 

 of air is ample, a novice would naturally begin with 10 or 

 15 c.c. 



* Rayleigh & Ramsay, Phil. Trans. 186, p. 224 (1895) ; see also Nature, 

 vol. 52, p. 163 (1895). " 



