The richness of the poem's visual imagery is seen when the speaker describes what is conjured in his mind when he hears the solitary reaper's song.  Wordsworth has to communicate to the reader the experience of sound.  This can only be done through employing rich visual imagery, so that the reader can experience the song in terms of what is conjured in the mind.  It is here in which the poem becomes rich with visual imagery.  The visual imagery of nightingales, Arabian sands, bodies of water, and the Hebrides all represent imagery that triggers mental pictures in the reader's mind.  As the song invokes these images in the speaker's mind, the rich imagery extends to the reader, who is also transported to a world beyond what is into what could be.  The rich visual imagery in the poem is designed to showcase the transformative capacity of the song. This is where the poem features rich imagery of a visual capacity.  In order for the solitary reaper's song to be appreciated, the imagery that Wordsworth uses has to be rich from a visual point of view.