Given a real number $ c > 0$, a sequence $ (x_n)$ of real numbers is defined by $ x_{n \plus{} 1} \equal{} \sqrt {c \minus{} \sqrt {c \plus{} x_n}}$ for $ n \ge 0$. Find all values of $ c$ such that for each initial value $ x_0$ in $ (0, c)$, the sequence $ (x_n)$ is defined for all $ n$ and has a finite limit $ \lim x_n$ when $ n\to \plus{} \infty$.
1. **Determine the domain of the sequence:**
   The sequence is defined by \( x_{n+1} = \sqrt{c - \sqrt{c + x_n}} \). For this to be defined, the expression inside the square root must be non-negative:
   \[
   c - \sqrt{c + x_n} \geq 0 \implies \sqrt{c + x_n} \leq c \implies c + x_n \leq c^2 \implies x_n \leq c^2 - c
   \]
   Since \( x_0 \in (0, c) \), we need \( c \leq c^2 - c \). Solving this inequality:
   \[
   c^2 - 2c \geq 0 \implies c(c - 2) \geq 0
   \]
   Since \( c > 0 \), we get \( c \geq 2 \).

2. **Analyze the limit of the sequence:**
   Suppose the sequence \( (x_n) \) converges to a limit \( L \). Then:
   \[
   L = \sqrt{c - \sqrt{c + L}}
   \]
   Squaring both sides:
   \[
   L^2 = c - \sqrt{c + L}
   \]
   Isolate the square root term:
   \[
   \sqrt{c + L} = c - L^2
   \]
   Square both sides again:
   \[
   c + L = (c - L^2)^2
   \]
   Expand and simplify:
   \[
   c + L = c^2 - 2cL^2 + L^4
   \]
   Rearrange terms:
   \[
   L^4 - 2cL^2 - L + c^2 - c = 0
   \]

3. **Solve the polynomial equation:**
   We need to find the roots of the polynomial \( L^4 - 2cL^2 - L + c^2 - c = 0 \). We factor it into quadratics:
   \[
   (L^2 + L - c + 1)(L^2 - L - c) = 0
   \]
   Solve each quadratic equation:
   \[
   L^2 + L - c + 1 = 0 \implies L = \frac{-1 \pm \sqrt{1 + 4(c - 1)}}{2} = \frac{-1 \pm \sqrt{4c - 3}}{2}
   \]
   \[
   L^2 - L - c = 0 \implies L = \frac{1 \pm \sqrt{1 + 4c}}{2}
   \]

4. **Determine the valid root:**
   Since \( 0 < L < \sqrt{c} \), we consider the positive root from the first quadratic:
   \[
   L = \frac{\sqrt{4c - 3} - 1}{2}
   \]
   This is valid if \( 4c - 3 > 0 \implies c > \frac{3}{4} \). However, since we already have \( c \geq 2 \), this condition is satisfied.

5. **Show convergence:**
   To show that \( (x_n) \) converges to \( L \), we use the fact that the sequence is bounded and monotonic. We have:
   \[
   |x_{n+1} - L| = \left| \sqrt{c - \sqrt{c + x_n}} - L \right|
   \]
   Using the mean value theorem and the fact that the function \( f(x) = \sqrt{c - \sqrt{c + x}} \) is continuous and differentiable, we can show that the sequence \( (x_n) \) converges to \( L \).

The final answer is \( \boxed{ c \geq 2 } \)