MLA

Wirgman, T. Blake. Robert Louis Stevenson at the Age of 27. 0AD. https://collections.library.yale.edu/catalog/2040801.

APA, 6th edition

Wirgman, T. B. (0 C.E.). Robert Louis Stevenson at the age of 27 [Drawings, Frontispieces (Illustrations), Illustrations, Prints (Visual works), Sketches]. https://collections.library.yale.edu/catalog/2040801.