diff --git a/doc/manual/figures/tangent.pdf b/doc/manual/figures/tangent.pdf
new file mode 100644
index 000000000..9b507aa04
Binary files /dev/null and b/doc/manual/figures/tangent.pdf differ
diff --git a/doc/manual/figures/tangent.svg b/doc/manual/figures/tangent.svg
new file mode 100644
index 000000000..db2b7bd3b
--- /dev/null
+++ b/doc/manual/figures/tangent.svg
@@ -0,0 +1,147 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   width="744.09448819"
+   height="1052.3622047"
+   id="svg2"
+   version="1.1"
+   inkscape:version="0.48.0 r9654"
+   sodipodi:docname="tangent.pdf">
+  <defs
+     id="defs4">
+    <marker
+       inkscape:stockid="Arrow1Lstart"
+       orient="auto"
+       refY="0.0"
+       refX="0.0"
+       id="Arrow1Lstart"
+       style="overflow:visible">
+      <path
+         id="path3938"
+         d="M 0.0,0.0 L 5.0,-5.0 L -12.5,0.0 L 5.0,5.0 L 0.0,0.0 z "
+         style="fill-rule:evenodd;stroke:#000000;stroke-width:1.0pt;marker-start:none"
+         transform="scale(0.8) translate(12.5,0)" />
+    </marker>
+    <marker
+       inkscape:stockid="Arrow1Lend"
+       orient="auto"
+       refY="0.0"
+       refX="0.0"
+       id="Arrow1Lend"
+       style="overflow:visible;">
+      <path
+         id="path3941"
+         d="M 0.0,0.0 L 5.0,-5.0 L -12.5,0.0 L 5.0,5.0 L 0.0,0.0 z "
+         style="fill-rule:evenodd;stroke:#000000;stroke-width:1.0pt;marker-start:none;"
+         transform="scale(0.8) rotate(180) translate(12.5,0)" />
+    </marker>
+  </defs>
+  <sodipodi:namedview
+     id="base"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:pageopacity="0.0"
+     inkscape:pageshadow="2"
+     inkscape:zoom="1.42"
+     inkscape:cx="266.63712"
+     inkscape:cy="493.35861"
+     inkscape:document-units="px"
+     inkscape:current-layer="layer1"
+     showgrid="false"
+     inkscape:window-width="1440"
+     inkscape:window-height="818"
+     inkscape:window-x="-2"
+     inkscape:window-y="24"
+     inkscape:window-maximized="1" />
+  <metadata
+     id="metadata7">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <g
+     inkscape:label="Layer 1"
+     inkscape:groupmode="layer"
+     id="layer1">
+    <path
+       style="fill:none;stroke:#000000;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;marker-end:url(#Arrow1Lend);marker-start:url(#Arrow1Lstart)"
+       d="m 574.28571,735.21933 -331.42857,-2.85715 2.85715,-331.42857"
+       id="path4569"
+       inkscape:connector-curvature="0" />
+    <path
+       style="fill:#ffffff;stroke:#000000;stroke-width:1.54900002000000003;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:4;stroke-opacity:1;stroke-dasharray:none"
+       d="m 242.5281,731.66507 c 0,0 8.76084,-156.45769 199.23956,-225.8869 21.78551,-7.94079 45.94809,-14.74315 72.74649,-20.04281"
+       id="path4941"
+       inkscape:connector-curvature="0"
+       sodipodi:nodetypes="csc" />
+    <path
+       style="fill:none;stroke:#000000;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1"
+       d="m 318.30986,580.5312 50.70422,-47.1831 0,45.77465 z"
+       id="path4945"
+       inkscape:connector-curvature="0" />
+    <path
+       style="fill:none;stroke:#000000;stroke-width:1.08317208px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1"
+       d="m 340.12098,560.90569 c 0,0 5.34212,3.26757 4.98598,17.97165"
+       id="path4947"
+       inkscape:connector-curvature="0" />
+    <flowRoot
+       xml:space="preserve"
+       id="flowRoot4949"
+       style="font-size:16px;font-style:normal;font-weight:normal;line-height:125%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;font-family:Sans"
+       transform="translate(53.521127,0.70422535)"><flowRegion
+         id="flowRegion4951"><rect
+           id="rect4953"
+           width="61.971832"
+           height="33.098591"
+           x="174.64789"
+           y="382.64386" /></flowRegion><flowPara
+         id="flowPara4955">σ</flowPara><flowPara
+         id="flowPara4957" /></flowRoot>    <flowRoot
+       xml:space="preserve"
+       id="flowRoot3078"
+       style="fill:black;stroke:none;stroke-opacity:1;stroke-width:1px;stroke-linejoin:miter;stroke-linecap:butt;fill-opacity:1;font-family:Sans;font-style:normal;font-weight:normal;font-size:16px;line-height:125%;letter-spacing:0px;word-spacing:0px"><flowRegion
+         id="flowRegion3080"><rect
+           id="rect3082"
+           width="45.774647"
+           height="27.464788"
+           x="585.21124"
+           y="742.50305" /></flowRegion><flowPara
+         id="flowPara3084"></flowPara></flowRoot>    <text
+       xml:space="preserve"
+       style="font-size:16px;font-style:normal;font-weight:normal;line-height:125%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;font-family:Sans"
+       x="583.8028"
+       y="741.79877"
+       id="text3086"
+       sodipodi:linespacing="125%"><tspan
+         sodipodi:role="line"
+         id="tspan3088"
+         x="583.8028"
+         y="741.79877">ε</tspan></text>
+    <flowRoot
+       xml:space="preserve"
+       id="flowRoot3090"
+       style="font-size:16px;font-style:normal;font-weight:normal;line-height:125%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;font-family:Sans"
+       transform="translate(-2.1126761,-4.2253521)"><flowRegion
+         id="flowRegion3092"><rect
+           id="rect3094"
+           width="17.605635"
+           height="18.309858"
+           x="350"
+           y="560.81287" /></flowRegion><flowPara
+         id="flowPara3096">D</flowPara></flowRoot>  </g>
+</svg>