206 Mr Purvis, On the influence of great dilution on the 



On the influence of great dilution on the absorption spectra of 

 highly concentrated solutions of the nitrates and chlorides of 

 Didymium and Erbium. By J. E. Purvis, M.A., St John's 

 College. 



[Bead 18 May 1903.] 



In a paper read by Professor Liveing before the Society and 

 published in the Transactions, Vol. xviii. p. 298 et seq., the 

 " Effects of dilution, temperature, and other circumstances on 

 the absorption spectra of solutions of Didymium and Erbium 

 salts" were described and discussed. The present paper is to 

 communicate the results of observations, completed some time 

 ago, when very highly diluted solutions were experimented with. 



Apparatus. 



The observations were made by the photographic plate, and 

 as the part of the spectrum studied was not in the ultra-violet, 

 no special lenses or prisms of calcite or quartz were used. The 

 spectroscope had a flint-glass prism of 60°, and fixed on the stage 

 at the minimum angle of deviation for the sodium yellow line. 

 The image was focussed on the photographic plate by an achro- 

 matic lens, and its length was about 3 inches. The glass 

 tubes which held the solutions were 610 mm., 1224 mm., and 

 3599 mm. long respectively, and each of 16 mm. internal 

 diameter. The first and second were fitted at each end with 

 glass plates in the manner described in Professor Liveing's paper. 

 Each of these tubes had a short upper branch placed about the 

 centre of its length through which it was filled. The long tube 

 of 3599 mm. length was provided with screw caps by which the 

 glass ends could be firmly held between rubber washers against 

 the two ends of the tube. Duplicate tubes of exactly the same 

 length and shape were used in the case of the two tubes of 

 610 mm. and 1224 mm. length. In all these tubes the circular 

 openings at the ends had exactly the same diameter. Heavy 

 wooden blocks were used as supports for the tubes, and they were 

 rigidly held in position on the bench by means of right-angled 

 stays which firmly gripped the lower portion of each block and 

 were glued to the bench. Along the upper surface of each block 

 a V-shaped groove was cut in which the tubes rested, and so that 

 they could be easily and exactly replaced in their former position 



