46 
LANGLEY. 
with an accuracy which ensures ns that we have not made 
an error so great as one-twentieth of a second. 
You see I hold in my hand a peculiar eye-piece, which 
has been made to observe this or any other terrestrial or 
celestial phenomenon of sudden occurrence. It can also 
be used for meridian observation, but its special field seems 
to lie in noting an event where no correction for personal 
equation is applicable. This event may be anything celes¬ 
tial or terrestrial, from the entrance of Venus on the disc of 
the sun to the explosion of a mine; but, for the purpose of 
illustration merely, let us take it to be the sudden appear¬ 
ance of the star. 
On looking into the telescope we see, in the first place, two 
prominent wires crossing each other at right angles, dividing 
the field of view into four quadrants. Now, by a simple 
mechanism, which I shall shortly explain, any object that 
our telescope is directed on—any fixed star, for example— 
seems to be revolving in the field, passing successively 
through the first, second, third, and fourth quadrants. If 
the star is hidden the mechanism is working just the same, 
and when the star appears it must evidently first be seen in 
some particular one of these four quadrants, and experience 
shows that we shall have no difficulty in telling in which 
one. The mechanism itself has recorded for us by an elec¬ 
tric contact the limiting instant between which it is possible 
to see the beginning and the end of the cycle during which 
revolution may be supposed to be made. It is not necessary 
that this cycle should last just a second; but, supposing it 
(still for illustration only) to be a second, if it was seen in 
the first quadrant, it was seen in the first quarter of the sec¬ 
ond ; if it was in the second quadrant, sometime in the sec¬ 
ond quarter of the second; in the third, in the third quar¬ 
ter; in the fourth, in the final quarter. All that we have to 
do in this case is to know in which second it occurred; for 
the quarter of a second we may say is noted for us by the 
purely automatic action of the optic lens and retina, since 
