Minor tweaks of the text

docs/source/introduction.rst

@@ -46,11 +46,13 @@ single 3D data volume.
 
    *Experimental geometry used in NXRefine.* 
 
-The Φ-axis is approximately perpendicular to the beam. The Φ-axis motor
-is on a χ-circle, with χ = 0° corresponding to a vertical axis of
-rotation. The figure shows a configuration, in which χ = -90°. The
-orientation of the Φ-axis can also be adjusted in the horizontal plane
-by ω and in the vertical plane by θ.
+The sample is at the center of a χ-circle, which can be rotated about
+the horizontal or vertical axes by θ or ω, respectively. When θ = ω = 0,
+the χ-circle is perpendicular to the incident beam. During a scan, the
+sample is rotated about the Φ-axis, which is vertical when χ = θ = 0.
+The figure shows a configuration, in which the Φ-axis is horizontal,
+with θ = ω = 0 and χ = -90°. The dotted lines show the orientation of
+the Φ-axis with ω = ±15°.
 
 .. note:: This geometry is equivalent to the four-circle geometry
           defined by H. You [see Fig. 1 in J. Appl. Cryst. **32**, 614