322 



Mr. T. C. Porter. 



[May 14, 



known ; and a rough estimate from observations of the brightnesses of 

 the pictures as usually projected by cinematographs on a screen would 

 point to as many as 50 per second being necessary where the arc light 

 is used for projection, though half this number would probably be 

 sufficient to prevent the nicker from being distressing. This assumes 

 that the photographs are "in register" on the screen. 



It has also an important bearing on the usefulness of a " nicker 

 photometer," for if I be the illumination to be measured, and n the 

 number of revolutions per second when nicker vanishes, and k and k' 

 constants, we have, as we have already seen, 



n = k log I + k'. 



Now the sensibility of the photometer depends upon the value of 

 dn/dl, which, from the above equation is equal to A/I, that is, it 

 diminishes as I increases, thus the photometer is not adapted for the 

 measurement of brilliant illuminations, at all events directly ; and it 

 must be remembered that, whether sudden or not, if there is a decided 

 change in the value of k in the above expression in the relation between 

 n and I at feeble illuminations, as fig. 2 indicates, the effect of this 

 will be to increase the probable error of a nicker photometer, for k 

 diminishes to practically half its former magnitude, hence the sensi- 

 bility of such a photometer for feeble illuminations will be diminished 

 by about one half. 



Finally, if " I " be the " last " of the impression undiminished, measured 

 in this experiment by the time the disc takes to make a half-re volution, 

 / == l/2n, and so I is given for the brighter illuminations considered by 

 the equation 



I = 1/(24*8 log I + 20), 



and for the fainter illuminations by 



I - 1/(3*12 log 1 + 53-25), 



the critical illumination being approximately that afforded by the 

 sperm candle already mentioned at a distance of 2 metres. 



Experiments were next made to determine how the rate of rotation of a 

 disc part white, part Mack, alters when the magnitude, of the white sector is 

 altered, the illumination of the disc by external sources being maintained 

 constant for each set of experiments. 



In giving an account of these experiments and in discussing them 

 it will be necessary to distinguish clearly between (1) the brightness 

 of the white sector at rest, which is independent of its angular breadth, 

 and depends solely on the distance of the disc from the source of 

 illumination, and on the intrinsic illuminating power of the latter ; 

 and (2) the apparent brightness of the disc when rotating and fiieker- 

 less, which depends not only on the external illuminating source (in fact, 



