The first $510$ positive integers are written on a blackboard: $1, 2, 3, ..., 510$. An [i]operation [/i] consists of of erasing two numbers whose sum is a prime number. What is the maximum number of operations in a row what can be done? Show how it is accomplished and explain why it can be done in no more operations.
1. **Identify the prime sum pairs:**
   - We need to find pairs of numbers whose sum is a prime number. 
   - Note that \(509\) is a prime number. Therefore, we can pair numbers such that their sum is \(509\).

2. **Pairing numbers:**
   - We can pair \(1\) with \(508\), \(2\) with \(507\), and so on up to \(254\) with \(255\). 
   - This gives us \(254\) pairs since \(254\) pairs are formed from \(1\) to \(254\) and \(255\) to \(508\).

3. **Remaining numbers:**
   - After performing \(254\) operations, the numbers \(509\) and \(510\) remain.
   - We can erase \(509\) and \(510\) since \(509 + 510 = 1019\) is a prime number.

4. **Total operations:**
   - The total number of operations is \(254\) (from the pairs) plus \(1\) (from \(509\) and \(510\)), which equals \(255\).

5. **Verification:**
   - We have used all the numbers from \(1\) to \(510\) in these operations, and no more numbers are left to pair.

Thus, the maximum number of operations that can be performed is \(255\).

The final answer is \(\boxed{255}\).