344 



Profs. Reinold and Rucker on the 



[June 21, 



Column VI. is obtained by subtracting the numbers in V. from the 

 corresponding numbers in IV. and dividing by the corresponding numbers 

 in II., and thus gives in megohms the resistance of a ring of the black 

 portion of the film 1 millim. in breadth. 



The numbers in Column II. are bracketed when they were obtained by 

 means of the same two series of measurements with the cathetometer, and 

 are thus dependent on the same observations. Numbers not bracketed 

 were obtained by totally independent optical and electrical measurements. 



It will be seen from this Table that five different films were studied 

 upon five different days ; and that, in all, 36 determinations of the resist- 

 ance of the black portions of the films were made. 



The highest and the lowest values obtained from individual experiments 

 differ from one another by about 11 per cent, of the mean value ; but the 

 means of the results of each day's observations display a closer agreement. 

 Thus the mean of the values obtained from 



Cylinder I. is 1-782, deduced from 19 observations. 

 „ II. „ 1-738, „ 1 „ 



„ III. „ 1-745, „ 4 „ 



„ IV. „ 1-693, „ 10 



„ V. „ 1-743, „ 2 



The maximum discrepancy is about 5 per cent, of the mean between 

 the greatest and least values. 



The figures, however, give interesting results when grouped in dif- 

 ferent ways. Thus, taking the means of the values obtained when the 

 lengths of the black part of the film lay between certain limits, it was 

 found that when the black part was 



> and < 2 millims., the mean value was 1-761, 



> 2 „ < 4 „ „ „ 1-764, 



> 4 „ < 6 „ „ „ 1-734, 



> 6 „ < 8 „ „ „ 1-760, 



> 8 „ <10 „ „ „ 1-756, 

 >10 „ <12 „ „ „ 1-760. 



Again, grouping the results according to the thickness of the coloured 

 portion of the film which appeared to be in immediate contact with the 

 black, it was found that when the colour of that portion, as viewed through 

 the telescope, was the 



blue of the second order, the mean value was 1-826, 



green „ „ „ „ 1-748, 



yellow „ „ „ „ 1-719, 



orange „ „ „ „ 1'756, 



red „ „ „ 1-716, 



green of third order, „ „ 1*738. 



