CHAPTER I.—INSTRUMENTS. 
7 
In order to use the photometer with a 6.5-inch reflector, a smaller tube was 
provided carrying an ocular and a diagonal reflecting plate to replace B. After 
several trials good images of the real and artificial stars were given by a diagonal 
plate with surfaces correct to one-tenth of a wave-length, furnished by Mr. O. L. 
Petitdidier, of Chicago. 
MAGNITUDES 
0123456 
Fig. 2.—Curve A 12. Absorption of Wedge V on 12- and 40-iNCH Refractors. 
The use of the instrument is very simple and convenient. The image of the 
star to be measured (shown at H in the drawing) is brought between the two 
images of the artificial star, and the wedge is moved by the pinion R till the light 
of the real star is matched by E, the image formed by reflection from the first 
surface of the plate B. The position of the wedge is then read on a scale divided 
to twenty-fifths of an inch, tenths of a division being estimated. If now the 
light of a star of known magnitude be measured, the only unknown quantity is 
the absorption of the wedge, expressed in magnitudes. 
