dazu eine zeitgenössische Quelle:
All about Binoculars, by Albert G. Ingalls. Scientific American,
August, 1944
"The first problem to be considered in the development of the
M13 from the basic M3 was that of waterproofing the instrument
to withstand submersion. This was accomplished by redesigning
the cover plates to provide for the use of a synthetic rubber
gasket and a greater number of fastening screws. In addition, a
new military wax, capable of withstanding extreme high and low
temperatures, for sealing the objective lens and objective
assembly was developed by the Ordnance Laboratory. This
compound, Specification FXS, replaces Navy Black Sealing
Compound No. 3A. It resists cracking at -50 degrees F. and has a
melting point of 210 degrees F., as against 150 degrees F. for
No. 3A compound. The formula includes a fungicide to repel molds
and insects."
