Thy glass will show thee how thy beauties wear,
Shakespeare's 77th sonnet repeats the theme of the passing of time and the inevitability of decay. There is less emphasis on the recuperative power of art than in some other works.
When you look in your mirror, you will see how quickly you are aging. This blank book will allow you to record the impressions of your mind, and these impressions will themselves teach you. The lines in your face that your mirror shows you will remind you of the open mouths of fresh graves. The hands of the dial will truly teach you how time keeps pushing you towards death. Write these things down now, and when you return to them much later you will find that they have matured along with you; they will mean more to you then. The oftener you do this, the more "your book" will profit by the discipline.
Source and analysis
Since George Steevens and Edmond Malone, the poem has been taken as referring to the gift of a blank-book or book of tablets, perhaps to the beloved, although some have suggested a more distant friendship than that in the other sonnets. Edward Dowden hypothesized that the poem relates specificially to the Rival Poet: knowing that he has lost favor, Shakespeare makes a present of this blank book to the beloved, who will now have to fill it himself, since Shakespeare has fallen silent.
The mirror and dial referred to in the sonnet are often assumed to be devices represented on the cover of the book; alternately, as Rolfe hypothesized, they might have been gifts enclosed with the book. Henry Charles Beeching discounts any clear biographical clue in the poem, arguing that it is so unrelated to those next it in the sequence that it must be read apart.