“1. Income derived by an individual who is a resident of a Contracting State from the performance of professional services or other activities of an independent character shall be taxable only in that State except in the following circumstances when such income may also be taxed in the other Contracting State:
“(b) if the individual's stay in the other State is for a period or periods exceeding in the aggregate 183 days within any twelve-month period commencing or ending in the year of income concerned or the basis period for the year of assessment, as the case may be; in that case, only so much of the income as is derived from such services or activities performed in that other State may be taxed in that other State.
“2. The term
‘professional services’ includes especially independent scientific, literary, artistic, educational or teaching activities as well as the independent activities of physicians, lawyers, engineers, architects, dentists and accountants.