Note 409 — ACT III.
Reading edition
Naturalists tell us the eagle holds up its brood, as soon as 'tis hatched, to the sun, to prove whether they are genuine or not. To this Shakespeare alludes, "Henry VI.," Third Part, act ii. sc. 1—
"Richard. "Nay, if thou be that princely eagle's bird, Show thy descent by gazing 'gainst the sun."