396 Messrs. R. W. Foray th and R. J. Sowter. 



one takes its place, situated between the other two (Plate 5, 

 fig. 10). This band is the most pronounced and the best defined 

 one in the whole series, and only becomes visible on the photo- 

 graphic plate in excessively dilute solutions, so dilute that one 

 might say the solution was colourless to the eye when viewed by 

 transmitted light. 



Phylloporpfiyrin and Hcematoporphyrin. 



On comparing the spectra of phylloporphyrin and haemato- 

 porphyrin in this region, and also those of their hydrochloric 

 acid compounds (Plate 3, figs. 6, 7, 8, and 9), it was found that 

 haematoporphyrin gave only a single band, but situated in the same 

 position as the double one of phylloporphyrin. On this point the 

 results of my experiments differ from those of Tschirch, who, as 

 stated above, found in both astw^Zeband occupying the same position. 

 In the hydrochloric compounds of hasmatoporphyrin, however, a 

 single band of the same pronounced character as that in phyllopor- 

 phyrin was found, the one in haBmatoporphyrin, as will be seen from 

 the figures, being slightly shifted towards the red end of the 

 spectrum, which is interesting from the fact that just in the same 

 way are the bands in the visible region of the spectrum of these 

 two compounds shifted, this constituting their only spectroscopic 

 difference. 



In conclusion, my thanks are due to Dr. E. Schunck and Dr. L. 

 Marchlewski for the valuable assistance they have given me in 

 many details in connection with this investigation. 



I hope in a further paper to investigate more particularly the 

 spectroscopic behaviour in the same region of the spectrum of the 

 yellow colouring matter accompanying chlorophyll in leaves and 

 allied colouring matters obtainable from other sources than the leaf, 

 for instance, carotin. 



" On Photographic Evidence of the Objective Reality of Com- 

 bination Tones." By R. W. FORSYTE, A.R.C.S., and R. J. 

 SOWTER, A.R.C.S. Communicated by Professor RUCKER, 

 Sec.R.S. Received March 29, Read May 5, 1898. 



[PLATES 6, 7.] 



In the following paper we propose to describe a series of photo- 

 graphs which prove the objective reality of difference and summation 

 tones. The work was suggested to us by Professor Riicker, and wehave 

 used the method of detecting these tones which has been described by 

 Riicker and Edser in the ' Philosophical Magazine ' for April, 1895. 





