A program of photographic photometry of the double quasar Q 0957+561 has been undertaken to study light variations in this pair of gravitational images and thereby determine the differential delay between the imaging paths. Measurements of plates taken during 1980 and 1981 reveal that whereas the northern image remained constant at a magnitude of 17.58 during this period, the southern image underwent a sudden outburst of amplitude 0.45 magnitudes, followed by a slow, nearly exponential fading. The characteristics of the event suggest it to be due to intrinsic variability in the imaged quasar, rather than effects in the lens galaxy. The differential time delay in the two image paths must thus be greater than 2.7 years. Examination of archive plates shows that both images were brighter in 1902-1915 than presently, demonstrating the existence of long-term variations as well.