222 PROCEEDINGS OF THE AMERICAN ACADEMY 



made, chiefly by Messrs. F. H. Crane and H. E. H. Clifford, students 

 in the laboratory, and careful and accurate observers. 



Five sets of observations on successive fusions of wire were made 

 by each observer, and the mean taken. The mean and the average 

 deviation from the mean of a long series of such sets were then found. 

 The tables give the results of the measurements. In all cases the 

 distance between the opposite lights is 78,74 in. (200 cm.), and the 

 diameter of the aperture in front of the gas flame f in. The recorded 

 distances are measured from the aperture. 



In order to give an idea of the degree of concordance of the separate 

 individual readings, the following sets (Table I.) are taken at random. 

 The nun,ibers given are the final positions of the disk, in each experi- 

 ment, in inches. 



TABLE I. 



Diameter of wire = .004 in. 



Observer M. Observer N. 



43.7 43.6 



43.6 43.7 



43.7 43.7 

 43.6 43.5 

 43.5 43.8 



Mean 43.62 Mean 43.66 



A change of 0.1 in. corresponds to an absolute change of 0.007 in 

 illuminating power of opposite light = 1.1 per cent. 



Diameter of wire = .006 in. 

 Observer N. Observer N. 



37.7 37.4 



37.8 37.2 

 37.4 37.4 

 37.2 36.8 

 37.2 37.2 



Mean 37.46 Mean 37.20 



A change of 0.1 in. corresponds to an absolute change of 0.01 in 

 illuminating power of opposite light = 1.0 per cent. 



Table II. is given at length as indicating more clearly the variations 

 among consecutive sets of measurements. I. P. denotes the illumi- 

 nating power of the platinum wire in terms of that of the opposite 

 light. The average deviation of I. P. is the change in this quantity 



