Cupid laid by his brand and fell asleep:
A maid of Dian’s this advantage found,
And his love-kindling fire did quickly steep
In a cold valley-fountain of that ground;
Which borrow’d from this holy fire of Love,
A dateless lively heat, still to endure,
And grew a seething bath, which yet men prove
Against strange maladies a sovereign cure.
But at my mistress’ eye Love’s brand new-fired,
The boy for trial needs would touch my breast;
I, sick withal, the help of bath desired,
And thither hied, a sad distemper’d guest,
But found no cure, the bath for my help lies
Where Cupid got new fire; my mistress’ eyes.
In plain English
Cupid fell asleep and left his torch unguarded. A follower of Diana found it and dunked the burning brand into a cold spring nearby. The spring absorbed love's divine heat and became a steaming bath—one that people still visit to cure all kinds of sickness.
But then my mistress looked at me, and Cupid reignited his torch in her eyes. The boy decided to test his weapon on my heart, and it worked: I became lovesick. I rushed to that famous healing bath hoping to be cured, but it was useless.
The real cure isn't in the water—it's in my mistress' eyes, where Cupid rekindled his fire. That's where I need to look if I want to heal, and that's the one place I can't escape.