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XVI 
ON THE DISTORTION OF PHOTOGRAPHIC STAR IMAGES 
DUE TO REFRACTION. By PROFESSOR ARTHUR A. 
RAMBAUT, M.A., D.Sc. 
[Read Aprit 19; Received for Publication Aprit 21; Published Juty 25, 1893.] 
Amonesr the principal advantages of the photographic method of 
making astronomical measures is the fact that, by its aid, much 
larger distances can be measured than is possible in the case of 
direct observations with any other instrument than the heliometer ; 
but when the distances over which the measures extend reach such 
large proportions as 2000” or 3000’—distances which are by no 
means uncommon in the study of astronomical photographs—the 
various disturbing causes, which affect the relative positions of the 
stars, become very much more effective than in the case of the 
shorter distances with which, up till recently, we have been accus- 
tomed to deal. 
The most important of these disturbing causes is the refrac- 
tion. 
In the “ Astronomische Nachrichten,’’ No. 3125, I have recently 
published formule, by which the correction for refraction to the 
relative position of any two stars on the plate may be computed 
in a manner which I have found in practice to be exceedingly 
convenient. Since, however, the stars are constantly changing 
their distance from the zenith all the time that the exposure of the 
plate continues, the correction for refraction is also constantly chang- 
ing, and a certain distortion in the shape of the images must take 
place, the amount of which will vary according to the altitude of the 
star and the length of time during which the exposure lasts. This 
distortion will, no doubt, be a small quantity; but in the more 
delicate researches of astronomy, such as the determination of the 
parallax of a fixed star, where we aspire to measure a quantity 
