164 Mr. H. E. Ives on the 



The correction for prismatic dispersion was determined by 

 measurements of the wave-length interval corresponding to 

 the width of the eye-slit, using the drum divisions and spectral 

 lines of known wave-length. 



The correction for prism absorption and for energy distri- 

 bution were determined together. Spectrophotometry com- 

 parison was made between the light of an incandescent lamp 

 previously compared with a black body of known temperature, 

 and the light of the tungsten lamp source through the ground 

 glass (3) and the prism (2). Employing the Wien equation 



14500 



J A = c 1 X" e~ at to determine the energy distribution for the 

 black body at the temperature measured, the distribution 

 of energy from the source as used is given in fig. 11 

 (PL III.). 



The calculated illuminations due to the various lamps placed 

 on the photometric bench (7) are subject to a reduction of 

 10*5 per cent, as determined by measuring the absorption 

 of the telescope lenses in a photometer. 



A large number of observations of the character described 

 were made almost daily during a period of about six months 

 by the writer, and by other observers as will be noted. 

 Illuminations were used whose corrected values are 1*8, 2*9, 

 8'9, 16-1, 26-9, 33, 68, 110, 175, 270 metre candles. These 

 illuminations, because of the artificial pupillary aperture of 

 1 sq. mm., correspond to lower illuminations in practical 

 photometry ; they will therefore be referred to here as 

 " illumination units." 



The observations were plotted on thin, rather transparent 

 coordinate paper, which permitted each observation to be 

 compared with any other by simply superposing and holding 

 to the light. This plan suited admirably; for in order to 

 make all the comparisons necessary, each curve had to be 

 studied in several different combinations. The same plan 

 is of course impossible in publication, so that it will be 

 necessary to select certain representative curves and to plot 

 several in each diagram ; the same curves will for that reason 

 figure several times. The necessity for this will be evident 

 from the following list of the combinations necessary for a 

 complete comparison with one observer alone. 



(1) Effect of changing illumination, for each of three field-sizes. 



(a) On the direct comparison curves. 



(b) On the flicker curves. 



(c) Relative position of the two curves due to the 



change. 



