Sonnet III

Look in thy glass and tell the face thou viewest
Now is the time that face should form another;
Whose fresh repair if now thou not renewest,
Thou dost beguile the world, unbless some mother.
For where is she so fair whose uneared womb
Disdains the tillage of thy husbandry?
Or who is he so fond will be the tomb
Of his self-love, to stop posterity?
Thou art thy mother's glass and she in thee
Calls back the lovely April of her prime;
So thou through windows of thine age shalt see,
Despite of wrinkles, this thy golden time.
   But if thou live, remembered not to be,
   Die single and thine image dies with thee.

 The youth is urged once more to look to posterity and to bless the world by begetting children. No woman, however beautiful, would disdain to have him as a mate. Just as he reflects his mother's beauty, showing how lovely she was in her prime, so a child of his would be a record of his own beauty. In his old age he could look on this child and see an image of what he once was. But if he chooses to remain single, everything will perish with him.

The 1609 Quarto Version

Looke in thy glaſſe and tell the face thou veweſt,
Now is the time that face ſhould forme an other,
Whoſe freſh repaire if now thou not reneweſt,
Thou doo'ſt beguile the world,vnbleſſe ſome
mother.
For where is ſhe ſo faire whoſe vn-eard wombe
Diſdaines the tillage of thy huſbandry?
Or who is he ſo fond will be the tombe,
Of his ſelfe loue to ſtop poſterity?
Thou art thy mothers glaſſe and ſhe in thee
Calls backe the louely Aprill of her prime,
So thou through windowes of thine age ſhalt ſee,
Diſpight of wrinkles this thy goulden time.
  But if thou liue remembred not to be,
  Die ſingle and thine Image dies with thee.

Commentary

1. Look in thy glass and tell the face thou viewest
glass = mirror; glass in the Sonnets usually means mirror.
the face thou viewest = your reflection. I.e. speak to yourself and tell yourself that 'Now is the time etc'.
2. Now is the time that face should form another;
I.e. by having a child.
3. Whose fresh repair if now thou not renewest,

If you do not undertake now the repair and renewal of your face, since it is fast decaying. whose refers back to the face thou viewest.

4. Thou dost beguile the world, unbless some mother.
beguile = cheat; deprive of its due rights.
unbless = make unhappy, deprive of fruitfulness, and the pleasure of being married to you.
some mother = some woman whom you might marry and cause to be a mother.
5. For where is she so fair whose uneared womb

For where is she so fair = what woman is so beautiful that; where is the woman in the world that (would be too proud to sleep with you).
uneared = unploughed. To ear is the old term for 'to plough', and often it is used meatphorically. As e.g. in Antony and Cleopatra:
Caesar, I bring thee word,
Menecrates and Menas, famous pirates,
Make the sea serve them, which they ear and wound
With keels of every kind.
AC.I.4.47-50.
where the keels are visualised as ploughing the sea.
uneared womb - The reference here is to sexual intercourse. Ploughing the womb, (as the plough enters into the soil so does the man enter into the woman), and sowing it with seed (semen) leads to children, as ploughing and sowing the land leads to crops. According to the physiology of the time, the male seed was the substance which created a child, and the woman was simply a carrier of the developing embryo. The biological details of reproduction were not understood. For the ploughing imagery compare:
He ploughed her and she cropped A.C. II.2.228
which is Agrippa's description of Julius Caesar's liaison with Cleopatra, which resulted in the birth of Caesarion.

6. Disdains the tillage of thy husbandry?
Disdains = is contemptuous of.
tillage of thy husbandry The farming and ploughing metaphor continues. Tillage is cultivation, working of the land; husbandry is farm and estate management, with a pun on 'being a husband'.
7. Or who is he so fond will be the tomb
fond = foolish

7-8 the tomb of his self-love in this context self-love leads to death, since there is no issue (i.e no children).
to stop posterity = to ensure that there are no descendants, to bring an end to future generations. The sentence has an additional sexual meaning, relating to masturbation. Onan was the biblical figure who was destroyed by God for spilling the seed 'that he might not have children'. See further commentary on SonnetI

8. Of his self-love, to stop posterity?
See above.
9. Thou art thy mother's glass and she in thee
Thou art thy mother's glass = you are effectively a mirror in which your mother can look to see a reflection of herself as she was in her youth.
10. Calls back the lovely April of her prime;

Calls back = recalls, remembers, brings back to mind.
the lovely April of her prime = her springtime, when she was most beautiful. April was the beginning of Spring, and was thought to be the most colourful of the months. Compare:

With April's first-born flowers, and all things rare
That heaven's air in this huge rondure hems.
21

11. So thou through windows of thine age shalt see,
through windows of thine age - This suggests not only looking back from old age, upon the past, as if through a window, but also looking at a child, one's own, as if seeing it through a window. The window can be both a barrier to and a point of contact with the world beyond.
12. Despite of wrinkles, this thy golden time.
Despite = in spite of.
thy golden time = the time of your golden youth, the time of your glory.
13. But if thou live, remembered not to be,

remembered not to be = determined not to be remembered, not being remembered. It ties in with the theme that the consequence of dying childless is to be erased from the book of memory.

14. Die single and thine image dies with thee.

If you die, as a single man, with no children, there will be no image to carry on your memory. The line could be read as a sort of tetchy imperative - 'Die as a single person then, if you must be so stubbornly inclined!'.