CHAPTER I.—INSTRUMENTS. 
9 
ard stars. To avoid these sources of error, wedge V, which was used in nearly all 
the measures here considered, was calibrated by the three following methods, 
in all of which the wedge was placed in situ, and the conditions in actual practice 
quite exactly used 1 : 
(1) Standard Pleiades stars, 2,700 settings. 
(2) Comparison with Zollner photometer, 3,000 settings. 
(3) Comparison with “wheel” photometer, 500 settings. 
Full details of this calibration will be published in connection with the work 
on “Determination of Standards for Faint Stellar Magnitude;” but in this con¬ 
nection a comparison of the results by the three methods will give the quantities 
used in the reductions and at the same time enable the reader to form an opinion 
of the degree of accuracy attained. 
(1) The standard Pleiades stars were measured with both the 6- and 12-inch 
telescopes, and the platted measures give the absorption in the second column of 
Table 3. 
(2) The Zollner polarizing photometer was set up in a dark room opposite the 
wedge photometer, and the artificial stars compared, first by projecting the 
Zollner star into the wedge, second by projecting the wedge star into the Zollner. 
The mean of these closely accordant results gives the absorptions in the third 
column of Table 3. 
(3) The wedge star was compared with an artificial star cut down by a revolv¬ 
ing wheel provided with sectors, giving the quantities in the fourth column. 
The weighted mean of these three determinations gives the values in column 
five, which are points in the absorption curve corresponding to the scale readings 
in the first column. This curve is called A 12, and is used in the reductions of 
the 12- and 40-inch measures. 
Since another ocular and diagonal plate was used in the 6-inch measures, 
and the appearance of the star images, both real and artificial, was different, 
it was not considered allowable to assume that the absorption curve would be 
the same; therefore the measures of the Pleiades stars were repeated with the 
6-inch and the comparison with the Zollner photometer was made with the same 
arrangement of apparatus as used on the 6-inch. The absorption curve found, 
called E 6, is shown by column six to differ enough from A 12 to justify the 
separate investigation, and at the same time to confirm the general features of 
the curve A 12. 
A few measures of faint stars were made in 1900 with wedge II, whose 
absorption curve derived from standard stars and the “wheel,” is given by 
points in the last column of the table. 
The question of the trustworthiness of the adopted absorption curve is of the 
highest importance in this work, and demands the most rigid scrutiny. The 
1 A fourth comparison with another polarizing photometer has since given identically the same 
absorption curve. 
