§1. When the notice of the convocation has been given legitimately, those present on the day and at the place determined in the same notice have the right to vote. The faculty of voting by letter or proxy is excluded unless the statutes legitimately provide otherwise.
§2. If one of the electors is present in the house where the election occurs but cannot be present at the election due to ill health, his or her written vote is to be sought by the tellers.
Source: Code of Canon Law (Vatican.va)