190 
ASTRONOMY: F. H. SEARES 
standards for the faint stars were especially desired, brighter objects 
have also been observed in order to facilitate comparisons with other 
photometric systems. 
A simple means of deriving a sequence of standards is to transfer 
to the region in question, by means of intercomparison photographs, 
the standards already established at the Pole. But in the present 
case this method could not be used; the range of brightness to be covered 
was too great, and the limiting magnitude to be attained too faint. 
Separate determinations of the scale were accordingly undertaken 
for each area, and, as a means of greater precision, these were based 
on all the stars shown on the photographs used. We have, therefore, 
for each region a large number of magnitudes, instead of merely se- 
quences of standards. The labor was thereby somewhat increased, but, 
besides the greater accuracy, there is the advantage of a complete utiliza- 
tion of the data of observation. 
To establish the photographic scale of magnitudes, the relation be- 
tween the brightness of a star and the size of its photographic image 
must be found. This can be reliably accomplished by means of succes- 
sive exposures, of constant duration, made on the same plate with light 
of different intensities. ^ The variations in intensity, which must be 
known, can. be produced through a change in the effective aperture 
of the telescope by adding diaphragms or screens of wire gauze. In- 
dividual stars will therefore show two or more images, with differing 
dimensions corresponding to known differences of magnitude. When 
once the images on any photograph have been completely measured, 
a simple interpolation process quickly gives the relative magnitudes 
of all the stars. To determine their absolute values, a zero-point 
correction must be added, whose value has to be derived from other data. 
If the scale is to be established over a considerable range of magnitude, 
photographs of both long and short exposure will be required; and to 
reduce the influence of the various errors, which in photometric work 
are always numerous and troublesome, several separate determinations 
will be made, preferably with diaphragms and screens which change 
the intensity by different amounts. The program adopted for each of 
the Selected Areas is shown in the Table. 
KO. PLATES 
APERTURE IN INCHES 
DXJRATION OF 
EXPOSURES 
1 
1 
2 
60, 32, 32, 60 
60, Screen, Screen, 60 
60, 32, 14, 9, 9, 14, 32, 60 
15^^ 
15 
2 
This arrangement of exposures admits of six separate determinations 
of the scale, and a possible maximum of sixteen different values of the 
