To summarize, there are enough inconsistencies in the appearance of the image to raise doubts that it represents a physical object. The most serious of these is that the image's sharpness appears in places to exceed the resolving power of the lens. This issue can and should be evaluated quantitatively. If the resolving power has, in fact, been exceeded, then the observation must be rejected as representing a photograph of an aerial disc. A hypothesis has been advanced to explain the image as a photographic artifact, proposing a method for its formation and a suggested test of its validity.

We are grateful to Marilyn Bruner for her reading of and technical observations on our paper "Photo Analysis of an Aerial Disk over Costa Rica." She raises several points which deserve further comment. These will be addressed in the same order as in her critique. We sympathize with her difficulty in making judgments about our analysis solely on the basis of a third-generation positive contrast print since its grain pattern might well misrepresent what is found on earlier generation negatives. In her first paragraph she suggested that she had the negative to study which she did not. We only had a second-generation negative to work from. Fortunately, a careful examination of the entire area on this negative under various levels of magnification provides the basis for several clarifications of the points she raises.

• We could find no significant change in grain size or spacing anywhere around the outside of this disk on this negative; this is what we said in the original text with regard to Figure 6.

• We agree that photographic density is high along the northerly boundary of the negative suggesting a high level of exposure. Of course this fact, by itself, does not point toward an optical artifact. Based upon optical density measurements alone, the brightest part of the disk is still lower in luminous intensity than sunlit clouds (i.e., < 13,500 ft-L).

• Another point she raises is the "abnormally sharp" step function of density on the northern edge of the image which, she states, is " much sharper, for example, than any physical feature on the coastline." This is true. However, careful inspection of the entire negative shows several roofs on houses having significantly sharper edges. This fact indicates clearly that the edge of the disk's image has not exceeded the resolution limit of the lens. It is unfortunate that Dr. Bruner could not have inspected the negative prior to making this observation.