oh ABSTRACT Attempts were made to detect at long distances the light emitted from the Ivy detonations. Two locations, Johnston Island (3,100 km) and Kwajalein Island (620 km), were chosen. At Kwajalein, io addition to a ground installation, equipment was also mounted in s plane flying above the cloud cover. sensitive photccells. ° Measurements were made with red-~ and blue- Out of a possible total of six records only one positive detection was made and this was from the equipment in the plane on the occasicn of King shut. Ine remaining cases are accounted for as follows: Three cases were timing notification uncertainties which mak it impossible to state tlat the equipment was operating at detonation time; one case where it is known that the equipmont was started after the detonation; and one case where the equipment was operating at the right time but there is no record of the light signal. It is concluded that light from a nuclear detonation can be detected to a distance of about 600 km under favorable conditions. Any further work should emphasiz* a basic study of the phenomena involved in the transmission of light beyond the horizon. ieeee ny a ee oe