 Barrett 's decline had a profound effect on Roger Waters ' songwriting , and the theme of mental illness permeated Pink Floyd 's later albums , particularly 1973 's The Dark Side of the Moon and 1975 's Wish You Were Here which was a deliberate and affectionate tribute to Barrett , the song , " Shine on You Crazy Diamond " , and also 1979 's The Wall . " Wish You Were Here " , partly about Barrett , borrows imagery of a " steel rail " from Barrett 's solo song , " If It 's in You , " from The Madcap Laughs album . 
