How could Rubashov's eyeglass be considered a motif in this novel?
Within the traditional definition of "motif" as it applies to literature, if we think of a motif as a recurring theme, fragment or event, then Rubashov's eyeglasses (known then as "Pince Nez" glasses) could be considered a motif.
The number of times he puts them on and off, both alone and during his lengthy and repeated interrogations reminds the reader, along with his well argued points, that Rubashov is an intellectual, a true ideological believer in the revolution, even after he has been arrested.
They can also be considered a symbol of dignity for a prisoner. A possession that is personal, functional and important, especially in the setting of a Soviet prison cell, which were known for being stark, sterile and forbidding places.