§1. If an infirm or elderly priest is unable to stand, he can celebrate the eucharistic sacrifice while seated, but not before the people except with the permission of the local ordinary; the liturgical laws are to be observed.
§2. A blind or otherwise infirm priest licitly celebrates the eucharistic sacrifice by using any approved text of the Mass with the assistance, if needed, of another priest, deacon, or even a properly instructed lay person.
Source: Code of Canon Law (Vatican.va)