energy is delivered to the eye before it can be protected by blinking. This is in contrast to low altitude detonations, where the delivery of thermal en- ergy is slower and the attenuation greater. The limiting horizontal distance at ground level for minimal burns was found to be 300 nautical miles for at 250,000 ft altitude and 225 nautical miles for at 140,000 ft. The size and severity of the lesion correlated with disfance. Correspondingly greater limiting distances would apply if the exposure was at altitudes where there would be proportionally less atmospheric attenuation All burna pro- duced within 160 nautical miles would have produced permanent injury or at least a segmented visual defect in man. Visual acuity would have been re- duced to from 20/100 to 20/200 if the lesion should occur on the macula. Projects 6.5 and 6.6 utilized ground based service radars to study the feasibility of determining fireball and cloud parameters, respectively, for very high altitude shots. Both had pursued similar objectives during earlier surface shots at Eniwetok. The returns from shot Teak were of short duration and did not appear until about H + 1 min, indicating initial absorption followed by reflection from the region of high electron density caused by the fireball. Although there are still attendant problems, location and yield determination for surface bursts using ground radars appears feasible; however, insufficient data were obtained to determine whether such detection is practical for very high altitude bursts. The cloud detection experiment during the earlier surface bursts was a continuation of work done on Operations Greenhouse, Redwing, and Plumbbob. It appeared that X-band radar was applicable to cloud detection for surface or near-surface bursts. For these, the range of detection is the line-ofsight distance, the detection duration is four to six tims longer in humid (EPG) areas than in arid (NTS) areas, and cloud parameters can be meas- ured within the accuracies of the equipment. For bursts above a 90,000-ft altitude, the lack of sufficient moisture or particle density and the level of electron density precludes detection of the cloud by X-band, ground based radars. Project 6.11 measured the absorption and induced ionization effects of very high altitude detonations with an aim toward resolving anticipated prob- lems of high powered ICBM detection radars. Five discrete frequencies from 10 to 1000 Mc were utilized in specially constructed radars. Riometers. were used to detect absorption by measuring any depressions in the integrated cosmic noise; with these, 30-, 60-, and 120-Mc frequencies were used. It was found that increasing the altitude of the shot radicaily increases the ionization and absorption effects in the region of 10 to 1000 Mc. Shot Teak, and to a lesser extent shot Orange, strikingly resembled a man-made auroral display observable both visually and with radio equipment, similar to the natural aurora that has been studied with the same techniques in Alaska. At altitudes such as that of shot Teak, considerable high frequency communication blackout occurs, and absorption on the order of minutes occurs near the shot even at ultra-high frequencies. Clutter from the shot-caused aurora would be of concern to a radar operating in the vicinity. Project 6.12 investigated the nature of radio frequency attenuation through the ionized region produced by very high altitude detonations. Rockets carried L- and S-band pulse carrier radio transmitters to above burat altitudes. Definite changes in signal were noted as the transmitters entered the region of the burst, although the signals were not completely lost. It was 42