18 Dr. H. E. Ives on 



6. Concluding Remakes. 



In surveying the results so far obtained, it must be borne 

 in mind that the present research is necessarily only of a 

 preliminary nature, owing to the restrictions imposed by the 

 optical means employed. Nevertheless, most valuable infor- 

 mation and experience have been gained, which will prove of 

 great help in planning the course along which future research 

 will have to be conducted. Thus the division of lines into 

 classes might profitably be carried still further by taking 

 into account the various types of enhanced lines, to which 

 I drew attention at the last meeting of the International 

 Union for Solar Research *. This, combined with an exten- 

 sion of the spectral region examined in the ultra-violet and 

 towards the longer wave-lengths, would, no doubt, bring in 

 many more line groups, and perhaps ultimately lead to the 

 discovery of the complete structure of the spectrum. The 

 grouping of apparently connected lines which has, so far, 

 been accomplished as a result of this investigation, may be 

 regarded as a first step in that direction. 



In conclusion, I desire to express my gratitude to 

 Sir Ernest Rutherford for the most generous hospitality 

 which he is extending to me in his laboratory, and to thank 

 him for the kind interest he is taking in the work. 



My thanks are also due to Professor Beattie, in whose 

 laboratory workshop many of the apparatus used in this 

 research have been constructed. 

 Manchester, Nov. 2, 1916. , 



II. Visual Difusivity . 

 By Herbert E. Ives f. 



1. Introduction. 



2. Application of the Idea of Diffusivity to Instantaneous Light 



Sources. 



3. Qualitative Observations on Retarded Images and After-images. 



4. Measurements. 



5. Discussion. 



1. Introduction. 



IjST two previous papers by the writer and Mr. E. F. 

 Kingsbury % on "The Theory of the Flicker Photo- 

 meter," the idea has been developed that the transmission 

 of impressions from the retina to the brain is in accordance 



* Bonn, 1913. 



f Communicated by the Author. 



t Ives and Kingsbury: "The Theory of the Flicker Photometer." — 

 I. & II. Phil. Mag., Nov. 1914, p. 708 ; April 1916, p. 290. 



