314 Messrs. H. E. Ives and E. F. Kingsbury on the 



and vice versa. The three curves of the figure then represent 

 the centre and two sides of such a field. At any given 

 speed flicker disappears, as the illumination from the varying 

 light is increased, first on the large-angle side, then on the 

 small-angle side, then in the centre. Effects of this sort 

 have been noticed by experimenters. 



6. Some Effects of Accidental Dissymmetry on the Critical 

 Frequency and Flicker Photometer Phenomena. 



Insistence has been made in previous papers on the im- 

 portance in the flicker photometer of avoiding all mechanical 

 flicker, such as that from black dividing lines between the 

 alternated fields. One of the most important experimental 

 results of the present work, and one largely unexpected, 

 has been to show in definite quantitative form the extreme 

 seriousness of any sort of lack of symmetry in the flicker 

 photometer. In fact, we believe that many of the contra- 

 dictory results obtained by experimenters are to be traced 

 back to negligence in the important precautions of securing 

 the greatest possible freedom from mechanical imper- 

 fections and in using a strictly substitution method of 

 observation. 



The manner in which these effects were discovered offers 

 the best way of presenting them. It was our original 

 intention to test the symmetry of the curve connecting 

 angular opening with critical frequency by alternating 

 observations, first on the illuminated rotating disk and then 

 on the illuminated surface F (fig. 5) as interrupted by the 

 same disk. By first equating the brightness of the disk and 

 white surface through the use of the disk edge to form an 

 equality photometer, it was thought that this alternate 

 measurement with one light and then the other would furnish 

 the best possible test of the symmetry of the effects of com- 

 plementary disks. 



The result of our first series of measurements so made, 

 with a disk of 30 degrees opening, is shown in fig. 12. 

 While the points obtained using the disk merely as an 

 opaque interrupter plot in the normal straight line form, 

 those obtained with the whitened disk fall on a well-marked 

 curve, partly above and partly below the straight line. 



The physical cause of this peculiar difference was at once 

 found to lie in the imperfect smoking of the disk with mag- 

 nesium oxide. A careful re-smoking practically cured the 

 trouble. But so much uncertainty appeared to us to be 

 possible as the character of the white surface varied with 



