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of smaller stars over a thread in the field of the telescope later than 
those of bright ones. 
I have now extended these researches by observations on the 
sudden disappearance of a luminous point at different degrees of 
brightness. 
The luminous point was produced by placing a copper plate with 
a small opening before the flame of a paraffin lamp. Its brightness 
ean be diminished by a wedge of coloured glass movable in a frame 
before the opening. Moreover a light metal screen at the further 
end of a lever, which can be moved quickly by means of a strong 
spring, was placed in front of the opening and in the researches 
on the appearance of the artificial star it uncovers the opening 
suddenly. At the same moment a metal point fastened to the screen 
dips into mercury and closes a current, thereby registering a 
mark on a chronograph. The observer at a distance of about 25 
meters from the apparatus, observes the appearance of the point by 
means of a telescope with a terrestrial eye-piece and at the moment 
when he perceives it, he closes with a key a second current which 
produces a mark on the same chronograph. The distance between 
the two marks will give the sum of the three foilowing periods: 
1st the time required to receive a perception of the luminous point; 
gnd the time elapsing between the perception and the closing of the 
current; 3° the difference between the times which elapse between 
the closing of the currents by the apparatus and by the observer, 
and the registering of the marks on the chronograph. 
For observations on the disappearance of the luminous point the 
arrangement remains the same except for the screen, which is 
replaced by one. of a different shape which suddenly covers the 
opening, when the lever is worked. 
Two Nicols of which one is fixed and the other can be turned, 
so that its rotation is measured by a graduated circle, are placed 
between the observer’s eye and the artificial star; they enable us to 
diminish its brightness in known proportions. If we determine at 
what reading of the graduated circle the brightness of the artificial 
star corresponds to the brightness of a star of known magnitude, 
scen through a telescope of given dimensions, we can express the 
different degrees of brightness in the corresponding star magnitudes. 
These determinations have been made by different astronomers of the 
observatory at Leyden who have taken part in the observations 
with the meridiancircle; they compared, from memory, the brightness 
of the luminous point at different positions of the Nicol with that 
