diff --git a/src/basics/vector.cc b/src/basics/vector.cc
index 3984a3c..e2913d5 100644
--- a/src/basics/vector.cc
+++ b/src/basics/vector.cc
@@ -42,6 +42,13 @@ Vector4::X()
     return mData[0];
 }
 
+const Double&
+Vector4::X()
+    const
+{
+    return mData[0];
+}
+
 
 /*
  * charles::basics::Vector4::Y --
@@ -53,6 +60,14 @@ Vector4::Y()
 }
 
 
+const Double&
+Vector4::Y()
+    const
+{
+    return mData[1];
+}
+
+
 /*
  * charles::basics::Vector4::Z --
  */
@@ -63,6 +78,14 @@ Vector4::Z()
 }
 
 
+const Double&
+Vector4::Z()
+    const
+{
+    return mData[2];
+}
+
+
 /*
  * charles::basics::Vector4::Length2 --
  */
diff --git a/src/basics/vector.hh b/src/basics/vector.hh
index 2ef9df4..4440e69 100644
--- a/src/basics/vector.hh
+++ b/src/basics/vector.hh
@@ -26,8 +26,11 @@ struct Vector4
     Vector4(const Double& x, const Double& y, const Double& z);
 
     Double& X();
+    const Double& X() const;
     Double& Y();
+    const Double& Y() const;
     Double& Z();
+    const Double& Z() const;
 
     /** Get the length-squared of this vector. */
     Double Length2() const;