Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Subtle

Subtle as a serpent.
—Anonymous

About as subtle as a sidewalk-worker for a second-hand clothing store.
—Anonymous

Subtle as the tone of the voice or the glance of the eye.
—John Burroughs

Subtle as a snake.
—Lord Lyttelton

A point as subtle
As Ariachne’s broken woof.
—William Shakespeare

Subtle as the fox for prey.
—William Shakespeare

Subtle as thin water.
—Algernon Charles Swinburne

Subtle as a serpent.
—Bayard Taylor

Subtle as light.
—Celia Thaxter

Subtle as a dead pig.
—Thomas Wright