The office or studio of a physician, surgeon, architect, dentist, teacher, painter or sculptor, musician, lawyer, accountant, engineer or consultant residing in the principal dwelling unit in which such office or studio is located, provided that there is no display or advertising on the premises in connection with such use and provided also that such studio or office does not occupy more space than 1/3 of the area of one floor of such principal dwelling, and further provided that such use is merely incidental to the use of such dwelling unit, and further provided that no more than one automobile carrying not more than two clients may ingress and temporarily remain on any lot at any one time in connection with said use, and provided, finally, that no assistants, whether paid or not, may participate in such use, except that one assistant may be employed if the nature of the profession is such as to require an assistant; provided, however, that such professional use shall not be deemed to include the right to engage in wholesale or retail trade.