In spite of this distraction, Rossi was able to complete, in 1933, an experiment on the east–west effect that he had begun before leaving Arcetri. Because this effect is more prominent near the equator, he organised an expedition to Asmara in Eritrea, which was then an Italian colony on the Red Sea at a latitude of 15° N. With Sergio De Benedetti, he set up a "cosmic ray telescope", which consisted of two separated GM counters in coincidence, whose axis of maximum sensitivity could be pointed in any direction. It soon became apparent that cosmic ray intensity from the West was significantly larger than that from the East. This meant that there was a larger influx of positive primary particles than of negative ones. At the time, this result was surprising, because most investigators held the preconceived notion that the primaries would be negative electrons.