Now I am not one of the most constant creatures alive myself, and am apt to run through the spectrum which has the blues at the bottom about once a week.

In a self-deprecating, conversational tone, the speaker admits to emotional inconsistency—being “not one of the most constant creatures alive.” The metaphor of “running through the spectrum” suggests cycling through a range of moods, with “the blues at the bottom” marking recurrent dips into melancholy. The line frames sadness not as an exceptional crisis but as a familiar, almost scheduled phase (“about once a week”), implying a temperament prone to periodic gloom. Its significance lies in normalizing fluctuating affect and presenting it with wit, turning private mood volatility into an intelligible, shareable image.