ON THE OBSERVATION OF SUDDEN PHENOMENA. 
BY 
S. P. Langley. 
[Read before the Society, March 2, 1889.] 
By a sudden phenomenon is here meant one of that large 
class where the occurrence is awaited without the observer’s 
previous knowledge of its exact instant, and of which familiar 
examples may be found in the bursting of a rocket, the ap¬ 
pearance of a meteor, or the emergence of a star from behind 
the moon. A great part of all the phenomena of daily life, 
as well as of scientific observation, are of this kind, though 
the importance of a special instance of another class (I refer 
to the gradual and foreseen approach of a star to a wire) has 
drawn to this latter such particular attention that we are 
apt to think only of it when “ personal error ” is in question. 
When in an observatory, we study the means taken to 
record the precise time of the transit of a star, we find that 
the precision of modern apparatus has reduced the error 
which we may expect in almost any part of the mechanism 
to an extremely minute amount, which may be calculated 
to the fractional part of the one hundredth of a second. I 
say “ almost,” for, as we are all aware, there is one notable 
exception, at least until photography can be made to inter¬ 
vene. The human brain and nerves, and behind these the 
inscrutable processes of the will, themselves form an inevit¬ 
able link in the chain of apparatus of observation, and here 
an error may and does arise, enormously greater than that 
of all the rest put together. 
We all know that this error varies with the individual 
and the occasion. It is most constant in the experienced 
4—Bull. Phil. Soc., Wash., Vol. 11. 
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