194 Scientific Proceedings, Royal Dublin Society. 
We thus conclude that, within the limits we have taken, so long 
as the zenith distance does not exceed 60°, no sensible error can 
arise through the distortion of a star by refraction if the measures 
are in all cases made from the centre of the image, and the 
coefficients in the formule of reduction are computed for the time 
corresponding to the middle of the exposure; but that if photo- 
graphs obtained with longer exposures are utilized for the determi- 
nation of the relative position of stars, it will be necessary to know 
what star on the plate was used as guider, and the distortion by 
refraction must be investigated for all stars at any considerable 
distance from it. 
