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



to avoid them. It must not, however, be gathered that their 

 occurrence renders the flicker photometer an unreliable or 

 troublesome instrument. It is quite as easy to use it 

 properly as improperly. 



7. Summary. 



While the primary purpose of this paper and the preceding 

 one has been to develop a theory to explain previously ascer- 

 tained experimental facts, the new experimental results — 

 called forth by the predictions of the theory and the wish to 

 verify all the consequences — are sufficiently valuable to 

 demand that the summary be divided into two parts — 

 " experimental" and "theoretical." 



Experimental. 



(a) Phenomena of frequency of disappearance of flicker. 



1. Different ratios of light to dark exposures under con- 

 stant illumination call for varying critical speeds. At high 

 illuminations a maximum occurs for approximately equal 

 light and dark intervals, but the curves connecting relative 

 period of light exposure and critical speed are not sym- 

 metrical. 



2. At low illuminations, using blue light, the maximum 

 at equal light and dark exposures is absent. Instead the 

 critical speed increases continuously as the ratio of light to 

 dark interval is decreased. 



3. The relationship between illumination and critical speed 

 is represented by the equation 



critical speed = a log illumination + b, 

 where " a " and " b " are constants, " a " being closely the 

 same whatever the relative light and dark exposures, and 

 " b " varying with the relative exposure. 



4. When a flickering illumination is superposed on a 

 steady one, the critical frequency is connected with the mean 

 illumination by the same logarithmic relation as that just 

 given, but the constant " a " varies with the ratio of flicker- 

 ing to steady illumination. 



(b) Behaviour of the flicker photometer. 



5. The high sensibility of the flicker photometer is shown 

 to be due to the very rapid increase in the critical frequency 

 of disappearance of flicker on each side of the equality 

 setting. 



6. In a flicker photometer which exposes the compared 

 colours for unequal periods, the less exposed colour will be 



