Again the novel touches on the theme that character, or "personality," is the unique, perishable element that gives value to the repetitive round of "action." Two kinds of light are posited in this paragraph, the light of communication, represented by Decoud's letter (and on a larger scale, the novel itself) and the "light of investigation." The latter, with its suggestion of material science and progress, reminds us of the light on the train that just passed.