langley] personal equation in transit observations 229 



overcome this objection a rearrangement of the apparatus was made 

 so that instead of intermittently obscuring the wires of the tally, 

 the wires were now continuously illuminated as in the usual method, 

 but the star itself was obscured by a shutter, except at the instants 

 when the wires had formerly been illuminated by the glow tube. 

 As thus modified the observer by this second method sees the star 

 only during well-separated intervals of a hundredth of a second or 

 less, so that all appearance of motion is avoided. This device, 

 suggested by Mr. Abbot, appears to be an improvement on the 

 former method, and has been used in the observations presented in 

 the accompanying table. These observations were made by three 

 observers, C. G. A., F. E. F., and N. E. G., of whom F. E. F. was 

 most experienced in observing transits without the personal equation 

 machine, and accustomed to its use by the former method, but less 

 practiced than the others with the machine as now employed. 



The general result appears to be in favor of the personal 

 equation machine. In the case of the observer C. G. A., whose 

 usual equation was about — o.io seconds when observing without 

 the machine, the error became only — 0.016 seconds with the 

 machine ; and the average deviation was reduced slightly. Sim- 

 ilarly with the observer N. E. G., whose direct observations were 

 visually rather variable, but which now yielded an average personal 

 equation of -j- 0.106 seconds, the employment of the machine was 

 of marked advantage, reducing his personal equation from 0.106 

 to 0.029 seconds and the average deviation of it from 0.080 to 

 0.033 seconds. With the observer F. E. F., the usual personal 

 equation was small and the average deviation of it was slightly 

 reduced. 



So far as it has been tried, then, the advantage of observing by 

 the aid of this device seems to be marked. 



