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