190 



STELLAR PHOTOGRAPHY. 



will be 0.0 for stars at any decimation from 0° 0' to 17° 15' inclusive. It will be 

 1.0 for stars between 65° 22' and 67° 39'. 



TABLE II. 



o 



o 



13 



88 30 1 



33 



38 1 



51 



45 1 



8 



52 1 



23 



58 1 



37 



89 3 1 



50 



8 1 



When a star is very near the pole, the cosine of the declination will be propor- 

 tional to the polar distance p. If this quantity is expressed in minutes of arc, 



the rate of motion will be proportional to 



P 



3438* 



The correction in magnitudes 



will then be 2.5 log .3438 



2.5 log p, or 8.84 — 2.5 log p. The correction for 



polar distances 1 



10 



100', and 1,000' will by this formul 



be 8.84, 6.34, 3.84 



and 1.34. A direct computation from cos 8 



g 



8.84, 6.34, 3.84, and 1.36 



O wing 



pre 



the 



brightness of 



o 



of th 



will vary 



from year to year. The effect will of course be very small, except for stars near 



pol 



As shown above, the correction for declination is 2.5 log cos 8 



The 



differential coefficient of this 



exp 



with 



spect to 8 



2.5 X .434 tan 8, or 



1.085 tan 8 



If the precession in declination for n years 



20 ".45 n cos « 



206265 



Ml 



the chang 



the correction is .000107 n cos a tan 8 



A table of 133.7 sin a tan 8 



is 



g 



in Oeltzen (Vol. I. p. xiii) for various 



values of a and 8. This table may be employed for the present purpose by 



plac 

 and 



the 



expression 



obtained in the form .0000008003 (133.7 sin a tan 8) 



ging a by 6 hours 



The sign is determined by the 



rule that for stars 



between 6* and 18* of right ascension the correction to the photographic brightness 



diminish 



For stars in the immediate 



nity of the pole, when the pol 



distance ex- 



pressed in minutes 



p, tan 8 



3438 

 P 



and the correction for precession 0.37 



n cos a 



P 



