82 Prof. F. Allen on the Persistence of 



sections horizontal, thence through the spectrometer (F), 

 and was finally viewed in an eyepiece in which all the light 

 of the spectrum, except a narrow central band or! any 

 desired colour, was cut off by means of adjustable shutters. 



V? F 



[^ 



u 



In making the measurements, the eye, which before each 

 reading was always rested in ordinary daylight, observed any 

 selected patch of colour which was caused to flicker by the 

 rotation of the sectored disk. The speed of the disk was 

 increased until the critical frequency of flicker was reached, 

 and then electrically recorded on a chronograph. 



At first the principal sections of the nicols were parallel, 

 so that a persistency curve might be obtained for the greatest 

 intensity of light possible with the apparatus. It may be 

 pointed out that any error arising from the use of Nicol prisms 

 in colour measurements does not affect these results, as they 

 are purely comparative. 



On subsequent days similar curves were obtained for 

 spectra whose intensities were diminished in determinate 

 amounts by rotating the polarizer through suitable angles. 

 In all, nine curves were determined for a series of luminosities 

 varying from the fullest brightness of the spectrum down to 

 the point where very little light was perceptible. 



In Table I. the first column shows the wave-lengths for 

 which determinations of the duration of the luminous im- 

 pression at the critical frequency of flicker were made, viz. 

 \ -725 fj., X -665 ft, X '590 fi, A, '520 /a, \ *460 p, and X -430 //, ; 

 the other wave-lengths in the table, and the corresponding- 

 data, were obtained by inspection from the curves formed 

 by the first six experimental values. The non-experimental 

 data are placed in the table to indicate the lowest points on 

 the respective persistency curves, and hence denote the 

 brightest points in the corresponding spectra. The numbers, 

 0°, 20°, 40°, &c., at the heads of the pairs of columns in the 

 table, are the angles between the principal sections of the 

 nicols from which the relative brightnesses of the corre- 

 sponding spectra are computed. 



