Ruskin presumably is alluding to lines included in Samuel Johnson 's The Vanity of Human Wishes:
See nations slowly wise and meanly just,
To buried merit raise the tardy bust.
(Johnson, The Vanity of Human Wishes, ll. 161-2 in Johnson, Complete English Poems)
These lines are quoted in Johnson 's Dictionary under 'just,' with the following footnote appended:
A reference to the bust of Milton, placed in Westminster Abbey in 1737 (Prologue to Comus, ll. 21-24), but also a general reflection on such tardiness as shown in the erection of monuments in the Abbey to Dryden in 1720, Butler in 1721, and Shakespeare in 1741.