190 
RESEARCHES in steelar photometry. 
The great thickness of the 40-inch objective, about 4} inches, requires a 
correction for absorption. Mr. Lundin informed me that the Clarks estimated 
that 85 per cent of the light was transmitted by the lens. The corresponding 
correction is 0.18 magnitude, reducing the calculated limit to 16.83, in good 
agreement with the observed values. 
The 275 ocular used on the 12-inch was made by Steinheil and is of good, 
but not unusual transparency, showing a limit in good agreement with the 
calculated value. 
The 6-inch requires special mention, since it is a silvered-glass Newtonian 
reflector. This form is usually credited with a reflecting power very much lower 
than these measures indicate. For example, we read in Young’s General Astron¬ 
omy, paragraph 49: “In a Newtonian reflector in average condition, the percent¬ 
age (reflected to incident light) seldom exceeds 50 per cent, and more frequently 
is lower than higher.’’ If this were correct the limit should be 12.3 magnitude, 
while in fact the reflector does nearly all that could be expected of a refractor of 
equal aperture, in spite of the fact that the mirror was silvered only three times 
in fourteen years, in 1892, 1897, an d I 9°3- 
It should be noted as a matter of greatest importance that these limits of 
vision furnish a check on the adopted absorption curve of the photometer wedge 
used in the work. A change of 5 per cent in the curve would make a difference 
of half a magnitude in the range between the standard 7th magnitude stars and 
the limit of the 40-inch. It seems justifiable to conclude, therefore, that the 
absorption curve is not in error by as much as 5 per cent. 
Table 119- Limit of Vision. 
6-INCH, OCULAR 40 . 
Magnitude H. 
Reduction. 
Corrected Magnitude. 
Field. 
Date. 
Estimate 
Star. 
Limit- 
z. 
Cor. 
H. 
P. 
y m d 
T Andromedae ... 
94 ii 20 
2 <x 
13.12 
13.40 
21° 
.01 
I 3 - 4 I 
13-49 
R Comae. 
94 5 3 i 
b 
12.29 
12.29 
43 
.08 
13-37 
13 67 
R Comae. 
04 5 10 
z 
12.83 
12.83 
34 
.04 
12.87 
13-17 
SX Cygni. 
04 11 30 
n 
11.84 
11.84 
37 
•05 
11.89 
12.24 
W Andromedae... . 
04 10 6 
h 
11.80 
11.80 
22 
.01 
11.81 
12.22 
Mean. 
12.71 
12.96 
OCULAR 80 . 
T Andromedae . .. 
96 10 24 
2-3 <k 
11.85 
12.20 
40 
.06 
12.26 
12.34 
V Andromedae. . .. 
98 10 11 
4 <d 
11.83 
12.35 
46 
.09 
12.44 
12.73 
RU Herculis. 
97 7 23 
P 
13-42 
1342 
34 
.04 
I 3-46 
13-73 
RV Herculis .... 
98 8 24 
l 
13.0 
13.0 
26 
.02 
13.0 
13-25 
RV Herculis. 
98 8 22 
l 
13.0 
13.0 
58 
. 20 
13.2 
13-45 
Mpan. 
12.67 
13.10 
