Relative Visibility of Different Colours of the Spectrum. 715 



The scale of the a, activities was chosen so that the experi- 

 mental point 220 hours from the start lay on the theoretical 

 curve. It will be seen that, while the curves do not agree 

 for the first few hours, the subsequent close agreement esta- 

 blishes the fact of the direct growth of radiothorium from 

 the mesothorium 2. 



Earlier experiments showing a greater initial drop in the 

 activity were shown by the absorption-curves to be due 

 largely to ThB and ThC present. Farther observations 

 were made of the activity during the first few hours after 

 precipitation, when the electroscope was filled with hydrogen. 

 This minimized the effect of the large quantity of /3 and 

 y rays present, and increased the effect of the small a present. 

 The curve obtained showed an initial activity relatively 

 slightly less than that obtained in air, but was otherwise 

 similar. 



The difference-curve being approximately an exponential 

 curve with the period of mesothorium 2, suggests the possi- 

 bility of some of the atoms of mesothorium 2 disintegrating 

 with the emission of an a ray. The number of these, how- 

 ever, must be very small, and is shown by these curves to be 

 less than 3 in 10,000. Owing to the similarity of period of 

 mesothorium 2 and thorium B, it is difficult to make quite 

 sure that the initial decay of the a rays occurs with the same 

 period as that of mesothorium 2, and the point is left open 

 for further examination. 



I desire to thank Mr. F. Soddy, M.A., F.R.S., for sug- 

 gesting this research, for the interest he has shown in it, and 

 for the use of materials employed in carrying it out. 



Physical Chemistry Laboratory, 

 Glasgow University. 



LXXV. The Relative Visibility of the Different Colours of the 

 Spectrum. Being a Report of a Committee on that subject 

 appointed by the Physical Society of Glasgoiv University* . 

 By R. A. Houstoun, Ph.DA 



IT is well known that light in the middle of the spectrum 

 is much bripjhter than light at the ends for the same 

 intensity of radiation in ergs/sq.cm.sec. The brightness of 



* The Physical Society of Glasgow University is a students' society. 

 The experiments described in the above paper were taken up by a 

 committee, and the work was done by the following : — Miss Margt. 

 B. Moir, M.A., B.Sc, and Messrs. A. E. Brown, M.A., B.Sc, John 

 Brown, M.A., B.Sc, F. F. S. Bryson, M.A., and A. S. Morrison, 

 M.A., B.Sc. 



t Communicated by Dr. B. A. Houstoun. 



