[D] We wish to thank Professor C. A. Maney and Captain W. B. Nash for their generous help with this problem. Although they do not agree with our conclusions, Professor Maney has kindly made available certain useful documents and Captain Nash in a lengthy correspondence has patiently answered a great many questions of detail. When puzzling observations in a laboratory seem to point to a conclusion that contradicts the main body of scientific knowledge, the researcher first tries to repeat the experiment and duplicate the observations. If this is impossible, as with the Chesapeake Bay phenomena, he next re-examines the assumptions on which the conclusion is based. The belief that the UFOs had an extraterrestrial origin is based chiefly on two assumptions: first, that the estimates of the disks’ size, distance, and speed were reasonably accurate; and second, that the disks were solid objects. If either assumption is unsound, the extraterrestrial theory is unnecessary and the incident becomes much less of a puzzle. Both witnesses were able and experienced observers. Nevertheless their determinations of distance and size, and hence of speed, are open to question because of the very fact that the disks were unidentified phenomena. Angular estimates are usually reliable when an observer is judging the position and speed of other known aircraft moving in the sky. But when the moving object is a strange one and is seen against an empty sky or flat ground containing no standards of comparison, estimates of actual size mean very little. The ability to judge distance depends largely on the binocular vision of the observer’s eyes, separated by a span of about 2.5 inches. Focused on an object at 300 feet, they subtend an angle of about one fortieth of a degree, less than one tenth the diameter of the full moon. This is a physiological fact, and means that if the observer is more than 300 feet away from an object of unknown size, he cannot determine its distance accurately unless he knows how large it is or unless he can compare it with a known object. Using angular estimates, the witnesses in the Chesapeake Bay case calculated that at the point of closest approach the disks were a mile lower than the plane and about half a mile to the north--a distance of roughly 7000 feet. Mentally comparing their appearance with that of a DC-3 aircraft at this distance, the observers arrived at an estimate of size--whose accuracy depends on having a known distance. The circularity of this process indicates the weakness of all the estimates given. Even the most skillful observer cannot accurately judge the distance of an unidentified object when he does not know its true size, and he cannot judge the size unless he knows its actual distance.