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1. Income derived by a resident of one of the States in respect of professional services or other activities of an independent character shall be taxable only in that State unless he has a fixed base regularly available to him in the other State for the purpose of performing his activities. If he has such a fixed base, the income may be taxed in the other State but only so much of it as is attributable to that fixed base. A resident of one of the States performing such professional services or other activities of an independent character in the other State shall be deemed to have such a fixed base available to him in that other State if he is present in that other State for a period or periods exceeding in the aggregate 183 days in any twelve month period.
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.