ASTRONOMY: F. H. SEARES 
311 
single-thickness screen, 60 inches in diameter, and the 32, 14, 8, and 
6-inch diaphragms, besides two other arrangements of less importance. 
For the interval 10.5-17.6, the mean of 17 scales with the 32-inch 
diaphragm is practically identical with that of 10 foimd with the 14- 
inch diaphragm. The 5 scales with the two smaller diaphragms show a 
deviation of about 1% from the mean of all, and there is a similar di- 
vergence between the results for the wire gauze screen and the mean for 
all the diaphragms. The agreement is therefore such as to leave little 
doubt as to the neghgible character of the diffraction effect and the 
satisfactory elimination of other errors. 
The longer exposure plates, which in several cases reach the nine- 
teenth magnitude, were all obtained with diaphragms of either 32 or 14 
inches, and are Hkewise satisfactorily accordant. These plates also 
give results for the brightest of the intermediate stars; in other words, 
they overlap completely the region covered by the plates of shorter ex- 
posure. The average difference between the mean scales from the 
two series, derived from nine groups of stars between 10.6 and 16.8, is 
0.015 mag. There seem, therefore, to be no systematic errors which 
depend upon the exposure time. 
The determination of the scale for the two remaining groups — faint 
stars and bright stars — presupposes a knowledge of that for the inter- 
mediate group. Here again the methods have been fully described 
elsewhere,^ and only a very brief statement need be included here. 
The extension to the fainter objects was by means of plates which, 
with one exception, received two exposures, both with the full aperture 
of 60 inches, but with different exposure times; the longer exposure was 
four or five hours, the shorter approximately half an hour. The reduc- 
tion was based upon the empirical relation estabhshed by Kron,^ which 
expresses the law of photographic action. Strictly speaking, the process 
involves an extrapolation, but one which is fairly rehable. The accord- 
ance of the six plates for the faint stars is scarcely less satisfactory than 
that of the long-exposure diaphragm plates used for the intermediate 
stars. The limiting photographic magnitude thus reached is about 20, 
which may be taken as the faintest attainable, under favorable conditions, 
with an exposure of four hours. 
Owing to the small field of the reflector, and the relatively wide dis- 
tribution of the bright stars, these objects had generally to be observed 
individually. They were photographed with screens or diaphragms 
producing images comparable with those of stars between the tenth and 
fifteenth magnitudes, obtained with the same exposure but without screen 
or diaphragm. The following is a typical arrangement of the exposures, 
