The magnifying power of the segment of a glass sphere had been noted by
Alhazen, who had observed also that the magnification was increased by
increasing the size of the segment used. Bacon took up the discussion of
the comparative advantages of segments, and in this discussion seems to
show that he understood how to trace the progress of the rays of light
through a spherical transparent body, and how to determine the place of
the image. He also described a method of constructing a telescope, but
it is by no means clear that he had ever actually constructed such an
instrument. It is also a mooted question as to whether his instructions
as to the construction of such an instrument would have enabled any one
to construct one. The vagaries of the names of terms as he uses them
allow such latitude in interpretation that modern scientists are not
agreed as to the practicability of Bacon's suggestions. For example, he
constantly refers to force under such names as virtus, species, imago,
agentis, and a score of other names, and this naturally gives rise
to the great differences in the interpretations of his writings, with
corresponding differences in estimates of them.

The claim that Bacon originated the use of lenses, in the form of
spectacles, cannot be proven. Smith has determined that as early as the
opening years of the fourteenth century such lenses were in use, but
this proves nothing as regards Bacon's connection with their invention.
The knowledge of lenses seems to be very ancient, if we may judge from
the convex lens of rock crystal found by Layard in his excavations
at Nimrud. There is nothing to show, however, that the ancients ever
thought of using them to correct defects of vision. Neither, apparently,
is it feasible to determine whether the idea of such an application
originated with Bacon.

Another mechanical discovery about which there has been a great deal of
discussion is Bacon's supposed invention of gunpowder. It appears that
in a certain passage of his work he describes the process of making a
substance that is, in effect, ordinary gunpowder; but it is more than
doubtful whether he understood the properties of the substance he
describes. It is fairly well established, however, that in Bacon's time
gunpowder was known to the Arabs, so that it should not be surprising
to find references made to it in Bacon's work, since there is reason to
believe that he constantly consulted Arabian writings.

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