56 S. P. Langley— Possibility of 
Under the second class come the ordinary procedures for the 
determination of relative error, or the personal equation between 
two observers; such as their interchange of stations, or their 
observing the same stars with adjacent instruments, or with the 
same instrument jointly, ete. These comparisons are liable to 
error themselves, are commonly tedious, and often compel long 
journeys. When successful, they give, not the absolute error, 
but the error as compared with another error, itself subject to 
an unknown variation. Unsatisfactory as all this is, however, 
such methods are in common use, and will remain common till 
better are provided. . An effort in this direction has stimulated 
the invention of a great number of devices to enable the ob- 
‘server to determine bis equation by comparison with the transit 
of an artificial star, itself, self-recording. This suggestion (said 
to have been sea made by Pr azmowski i os has been. the 
parent of many attempts, among which those of M.M. Hirsch 
and Plantamour; ane that of M. Wolf of ih Paris Observatory, 
particularly deserve attention; and of subsequent ones too 
numerous for description, though the apparatus of Professor 
Eastman, U.S. N., sais of Professor Rogers of Harvard me 
is fale? haa most are aware. 
To propose to offer anything new at this day on such a sub- 
ject may seem to be hardy. I venture however to ask attention 
to a method for eliminating the equation on the star itself, 
which has worked well on trial. 
Before stating it, Jet us observe that we restrict the words 
‘‘ personal equation” here, to the correction for the error com- 
mitted by the observer in noting the time of transit of a star 
owing to the motion of the latter, and that for brevity we leave 
untouched the effect of certain minute irregularities familiar to 
the observer, such as the actual inequality of wire eri 
which should be equal, or the apparent irregularity in the 
remove the error from the observation, in the act of observation 
itself. One mode which presented itself many years since, and 
which has been tried in numerous subsequent plans I have 
made, was to view the star only for a bnef time through a 
narrow linear aperture in a moving screen, thus exhibiting its 
