<a href="//commons.wikimedia.org/wiki/Gustav_Robert_Kirchhoff" title="Gustav Robert Kirchhoff">Gustav Robert Kirchhoff</a> (left) and <a href="//commons.wikimedia.org/wiki/Robert_Bunsen" title="Robert Bunsen">Robert Bunsen</a> (right)
Unknown (Courtesy Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library.)
circa 1850 <div style="display: none;">date QS:P,+1850-00-00T00:00:00Z/9,P1480,Q5727902</div>
<a rel="nofollow" class="external free" href="http://sceti.library.upenn.edu/sceti/smith/scientist.cfm?PictureID=2076&amp;ScientistID=158">http://sceti.library.upenn.edu/sceti/smith/scientist.cfm?PictureID=2076&amp;ScientistID=158</a>