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.
The
differential sharpness of the disk's image around its circumference is more difficult to explain, at least in terms
of a solid, three-dimensional object. One speculative explanation for the diffuse edge on the left side is that the
object is partially submerged so that the water interface produced an irregular boundary.
- Her comment that one would expect more light diffusion or halation around the bright disk than is found here is
interesting and raises a number of technical questions that requires far more space than is available to discuss.
Suffice it to say that there are several other objects in the field of view that are brighter than the disk which
possess extremely sharp edges (viz., toits de divers bâtiments). In none of these regions is there significant light
spillover from the roof area onto darker, adjacent areas of the film.
The absence of a shadow from the disk
remains a puzzle to us. As stated in our article, an obvious explanation is that the object is at the surface of the
earth where no shadow would be expected. Another possibility is that the object is opaque, small, and much nearer
the airplane so that its shadow's reduced size and darkness would be difficult or impossible to locate on the
ground.
- Her reference to light areas on the negative, that is, "the 'portholes' on the positive image" is unclear. We did
not use the term "porthole" or "portholes" and do not refer to any such areas. Perhaps she is referring to the
single circular shaped region at the approximate center of the disk which is a good deal lighter than the average
luminance of the disk (on the negative). That particular region is approximately the same density as is the surface
of the lake surrounding the disk.
