6 
RESEARCHES IN STEEEAR PHOTOMETRY. 
T 
an artificial star. In contact with G is a piece of blue glass to render the light 
of the star less yellow. An image of this star is thrown by the projecting lens 
P upon a plate of plane-parallel glass B and reflected 
from both surfaces into the focus of the ocular 0 , forming 
at E and F two images of the artificial star. Interposed 
jn the path of these rays is the photographic wedge W, 
movable at right angles to C by the rack and pinion R . 
The short tube carrying the ground glass G is movable away 
from the diaphragm D by means of the head of the screw S, 
projecting through an inclined slot in the farther side of 
the tube C. By this means the artificial star can be made 
larger and less sharply defined, thus resembling more closely a 
real star under different atmospheric conditions. Finally, 
a pair of shade glasses at A can be moved, either both or 
singly, into the path of the rays. 
In photometers made on this principle the all-important 
condition to be fulfilled is that the images of the real and 
artificial stars should closely resemble each other. The 
range of adjustment of the ground glass G was found to be 
insufficient to meet this condition with the different tele¬ 
scopes on which the photometer was to be used; therefore 
the diaphragm D, originally provided, which had a single 
aperture 0.17 mm. in diameter was replaced with a movable 
d iaphragm 
carrying four 
apertures, 
0.10, 0.15, 
0.20, and 
0.25 mm. in 
diameter. By 
choosing the 
most suitable 
aperture and 
c ombining 
with it a slight 
movement of 
the ground 
glass, it was 
possible to 
give the disk 
of the artifi¬ 
cial star any required size and sharpness to suit the various telescopes used and 
the different atmospheric conditions. With few exceptions the hole 0.15 was used 
with the 6- and 12-inch and 0.20 with the 40-inch. 
Sc a It - me hes 
Fig. i.—Section of Photometer. 
