The physician holds a urine glass (his own?) up against the candle. Death sneaks up from behind, places his left hand on the physician's shoulder and grabs the glass with his right hand.
This look very much like the dance of death in Bern (picture to the left). In Bern Death sneaks up from behind and strikes the physician's glass with a bone - breaking it and spilling the contents. The doctor clutches his crotch - as if he was ready to supply a new specimen.
In Holbein's picture Death doesn't break the glass, but instead seems to be handing it to the physician. In Holbein's great dance of death (picture below to the right), Death brings an old man to the physician and hands him a urine glass as a challenge.
Hidden in the right part of the picture — half-hidden by the letter — is a devil with chickens' feet and one more urine glass. Evidently it is in the process of filling another glass. In the very high resolution detail image to the left one can see how the devil is turning away and letting its "finny" arm rest on its lower back. In contrast the animation reveals this devil as a rooster with a cockscomb, so maybe the demon is a shapeshifter?
The accompanying text for this letter goes: »Der gelert stirbt zu glycher wyß wie der ungelert / vnd dorumb hat mich verdrossen mines lebens / do ich sach das all ding under der sonnen böss woren«.
This is from Ecclesiastes (Qoheleth) 2,16-17 which in Luther's version sounds: »[...] Wie stirbt doch der Weise samt dem Toren! Darum verdroß es mich zu leben, denn es war mir zuwider, was unter der Sonne geschieht [...]«. In English: »For there is no remembrance of the wise more than of the fool for ever; seeing that which now is in the days to come shall all be forgotten. And how dieth the wise man? as the fool. Therefore I hated life; because the work that is wrought under the sun is grievous unto me: for all is vanity and vexation of spirit«.