propose to discuss the two relevant communications from the point of view of 
the considerations enumerated above. 
From this point of view it is interesting to note that the use of the reseau 
was not new. It had been in use some time ago, but was abandoned until it has 
been re-introduced in recent years. The history of these changes and the under- 
lying reasons were given as follows (5): *By 1939 a camera... was available and 
the register glass was ruled with the grid of fine lines known as the reseau which 
was to be used to eliminate the film distortions." But „the work was completely 
interrupted by the war and not resumed until 1947. One mistake made was the 
too easy abandonment of the reseau.” This was attributed to an infection by ,,the 
epidemic of optimism which grew during the war about the stability of film, 
and since it was thought that the use of reseauinvolves morelaborious observa- 
tions (though this idea has since proven to be mistaken) much work was done 
with no elimination of film distortion other than those deduced from measurement 
of collimating marks." 
Two points may be noted in this connection: 
(a) that the optimism which led to the mistake of abandoning the use of the 
reseau. could only arise from inadequate experimentation; and (b) that this 
optimism most probably discouraged attempts to verify what was then current 
belief that observations with the reseau were more laborious. 
Situations like this happen only too often. The propaganda about the use 
of plotting machines in air triangulation is another example. The great emphasis 
that they offer tbe answers to all problems of air survey, including air triangula- 
tion, must have discouraged many a promising development of analytical methods. 
6.2. But let there be no doubt about the wisdom of the return to the use of 
reseau. The only necessary conditions are that it should not interfere with 
stereoscopic vision, and that it should not have an adverse influence on the 
quality of the photograph. Even if such undesirable effects resulted, it would 
still be necessary to weigh them against the advantages of using the reseau. This 
argument applies not only to the users of the analytical methods but to any user 
of the register glass type of camera, whether the reseau is intended for general use 
or only when intolerable distortions or other defects are suspected, and whether 
the reseau improves the accuracy much or little. 
On the other hand, users of other types of camera see the matter. from an 
entirely different point of view. To them it is a question of whether or not the 
improvement of accuracy is worth switching over to the register glass type. To 
the makers of other types of camera, a large improvement arising from the use 
of reseau may have serious repercussions on business! 
6.3. The published accounts would make the users of any but the register 
glass type of camera feel at a great disadvantage. There was therefore danger of 
breading another epidemic of optimism, or of pessimism depending on one's side 
of the controversy. 
The first assessment of the improvement due to the reseau was, as pointed out 
in Section 2.222, afflicted by a hidden difference between the methods used to 
compute the relative tilts on reseau and non-reseau overlaps respectively. This 
difference gave rise to an improvement in the case of the reseau photographs for 
which the reseau per se had no credit. 
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