diff --git a/sphere.cpp b/sphere.cpp
index 4419219a..827fc546 100644
--- a/sphere.cpp
+++ b/sphere.cpp
@@ -19,4 +19,14 @@
 using util::sphere;
 
 //-----------------------------------------------------------------------------
+template <size_t S, typename T>
+sphere<S,T>::sphere (point<S,T> _centre, T _radius):
+    centre (_centre),
+    radius (_radius)
+{
+    CHECK_GE (_radius, 0);
+}
+
+//-----------------------------------------------------------------------------
+template struct util::sphere<2,float>;
 template struct util::sphere<3,float>;
diff --git a/sphere.hpp b/sphere.hpp
index 7fb965e5..2896d64d 100644
--- a/sphere.hpp
+++ b/sphere.hpp
@@ -22,10 +22,13 @@
 namespace util {
     template <size_t S, typename T>
     struct sphere {
+        sphere (point<S,T> centre, T radius);
+
         point<S,T> centre;
         T radius;
     };
 
+    typedef sphere<2,float> sphere2f;
     typedef sphere<3,float> sphere3f;
 }