ASTRONOMY: F. H. SEARES 
31 
five additional photographs were made with single yellow exposures of 
128^ and 10°^, respectively, and corresponding blue exposures of 8% 
16% 32% 64% and I"', 2"^, 4"^. For these the full aperture of 60 inches 
was used, and the measurable images of all the stars shown were included 
in the discussion. On the plates of the first series described above, the 
images for objects of all degrees of brightness were approximately of the 
same size; but here there was no attempt at equalization. 
The question as to the effect of size of image upon the exposure-ratio 
had accordingly to be examined. Inasmuch as the gradation of a photo- 
graphic plate is to some extent a function of the wave-length of the 
light producing the image, the necessity for some correction of the 
exposure-ratios derived from these plates of longer exposure was antici- 
pated. Special photographs of bright stars taken with a wide range in 
aperture, but with the same series of exposure times, gave data for a 
provisional determination of the corrections. These seem to be moder- 
ate, unless the images used for finding the exposure-ratio are very large. 
The corrected mean differences are in the third and fourth columns of 
the table. The two plates of 10"" exposure were of another emulsion, 
requiring a different reduction curve, and were accordingly reduced to 
the standard curve which applies to the remainder of the data. 
Finally, an additional group of four plates of the original emulsion 
was exposed in November, four months after the others had been taken. 
The results for these are in the fifth column of the table. As the small 
correction depending on the size of the image is not yet well determined, 
it has been omitted in the case of this last group of plates. It will be 
noted that the results are substantially as before. 
Difference Between Color-Index from Magnitxjdes and that from Exposure-Ratio 
(Unit = 0.01 mag.; number of values in parentheses) 
PHOTOGRAPHIC 
MAGNITtDES 
DtJRATION YELLOW EXPOSURES 
WEIGHTED 
MEAN DtFFER- 
ENCE 
NUMBER OF 
STARS 
IN GROUP 
16, 32S 
128S 
16^ to 8.5™ 
3.5 
0(12) 
0 
2 
5.7 
-h3(16) 
+3 
3 
6.5 
-2(20) 
-2 
4 
7.5 
+2(20) 
+2 
3 
8.6 
-3(24) 
-3 
3 
9.4 
-7(18) 
-7 
3 
10.6 
-4(48) 
-4 
6 
11.4 
+5(54) 
+5(24) 
+4(14) 
-5(23) 
+3 
8 
12.3 
+2( 4) 
+3(15) 
-5(15) 
-1 
7 
13.3 
+6(25) 
+ 1(17) 
+4(25) 
+4 
12 
14.1 
0( 5) 
+3(26) 
-5( 7) 
+ 1 
13 
14.8 
0(26) 
0 
13 
15.5 
-6(21) 
+ 7( 6) 
-3 
11 
