It is certain, however, that in this part he describes the anatomy of the eye with great accuracy, and discusses mirrors and lenses.
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.