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., roof tops of various buildings). 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.