I'm not getting what Charles Lamb is saying in the following lines. If you kindly state your precious opinnion regarding this extract from Lamb's essay In Praise of Chimney Sweepers....

I am by theory obdurate to the seductiveness of what are called a fine set of teeth. Every pair of rosy lips is a casket, presumably holiding such jewels; but, methinks, they should take leave to "air" them as frugally as possible. The fine lady, or fine gentleman, who show me their teeth, show me bones. Yet must I confess, that from the mouth of a true sweep a display (even to ostentation) of those white and shinning ossifications, strike me as an agreeable anomaly in manners, and an allowable piece of foppery. It is, as when
A sable cloud
Turns forth her silver lining on the night.

Thanks in Anticipation