858 Mr. H. E. Ives on the 



of 3 and I transmission were used, so that these measure- 

 ments were actually made at lower illuminations than the 

 others. Since, however, the small field was used, and since 

 even these illuminations are only just getting into the region 

 of rapid change of luminosity curve shape (with large field), 

 the errors will be small. They are also in just the opposite 

 direction from the small errors due to the use of a wide slit 

 in this spectral region. 



The manner of taking observations was to start at the 

 middle of the spectrum, then take observations back and 

 forth, from red side to blue side, working toward the ends of 

 the spectrum. This was found a desirable procedure, because 

 it gave the observers a chance to obtain practice on the 

 measurements with least colour-difference. At the end the 

 first two or three readings were repeated until a close check 

 was found. With some observers a check was obtained on 

 the first wave-length, while with others there was a difference 

 of several per cent, until the second or third wave-length. 

 The mean of both sets was taken where a difference was 

 found. Such differences are probably to be ascribed to lack 

 of practice at the start. 



All the scale-readings were made by an assistant, who 

 watched voltages and held the speed constant. 



Observers. — Of the eighteen observers, the majority had 

 never used a flicker-photometer, while several had never 

 before looked in a photometer of any kind. It is a striking 

 point in favour of the flicker-photometer that its method of 

 use can be acquired so quickly that there is little choice 

 between the results of skilled and unskilled observers. The 

 former differ from the latter chiefly in the greater speed with 

 which they can make their readings. In instructing the 

 inexperienced observers the procedure was compared to 

 placing a blind man upon the side of a hill and asking him 

 to find the top. As he finds his position relative to the 

 summit only by stepping off in various directions, so the 

 photometrist locates the minimum of flicker only by oscillat- 

 ing the illumination (slit) until each direction of motion 

 increases the flicker. Standing still in either case is useless. 

 The correct speed was determined as that which made the 

 peak of the " hill" the sharpest. 



Results. 



The readings were first reduced to a normal equal energy 

 spectrum as described above. The resultant curves were 

 measured for area with a planimeter ; then all were reduced 

 to the same area and their mean taken. This seemed on the 

 whole the fairest way to average the results, in view of the 



