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



the same time. In the case of the tube of 3599 mm. length 

 there was no duplicate tube used ; but by means of the screw cap 

 it was found quite easy to fill the tube first with distilled water 

 and place the 5 mm. cell and its contents against one end as 

 described above. After a photograph had been taken, the water 

 was run out of the tube, and the latter was allowed to draiu 

 and dry, and then filled with the diluted solution of the salt, the 

 screw cap being carefully replaced in its previous position. 



It was found very essential to have the solutions absolutely 

 free from dust and gas bubbles. Again and again the results 

 were vitiated by these impediments ; for, although the solutions 

 appeared to the naked eye to be translucent and quite free from 

 dust and bubbles, the sensitive plate readily detected the loss in 

 the intensity of the light traversing them. These difficulties 

 were considerably lessened by regularly filtering the solutions 

 after several operations. 



With regard to the tubes of 610 mm. and 1224 mm. length 

 another arrangement was employed. It consisted in placing the 

 tube containing the diluted solutions at right angles to that 

 in front of the spectroscope, and so that the ray of light issuing 

 from it was reflected into the slit of the spectroscope by a right- 

 angled prism. The two sources of light were so arranged by the 

 eye and photographically that the intensities of the light entering 

 the spectroscope was exactly the same. By this arrangement the 

 absorption spectra of both the concentrated and the diluted 

 solutions were photographed at the same time. 



The absorption bands photographed Avere, in the case of the 

 Didymium solutions: — 



1. A weak band at about A, 496. 



2. A strong group of bands in the yellow at about A 590 — 



A570. 



3. A weak band at A, 531. 



4. A strong group in the green at about A528 — 520. 



5. A less strong group of two diffuse bands in the green 



whose centre is about A. 5 10. 



6. Three well-marked bands in the blue of about A, 483, 



A476, and A469. 



7. A broad weak band whose centre is about A 46 2. 



8. A broad band whose centre is about A 444. 



9. A weak band at about A 433. 



10. A narrow well-defined band at about A427. 



11. A weak band at about A418. 



12. A weak band at about A415. 



13. A weak band at about A406. 



14. A strong broad band at about A 403. 



Beyond this, the light was absorbed and the photographic 



