<story><title>Schlieren Optics – See small changes in the index of refraction in air</title><url>http://www.youtube.com/watch?v=mLp_rSBzteI</url></story><parent_chain></parent_chain><comment><author>pierrec</author><text>I&amp;#x27;ve long had this pet theory that soaring birds have a fine vision that allows them to see thermals (warm ascending currents) directly, through changes in the air&amp;#x27;s refraction index. I&amp;#x27;ve searched a bit, but no research seems to ever have considered the question. Just a wild theory, though.</text></comment>
<story><title>Schlieren Optics – See small changes in the index of refraction in air</title><url>http://www.youtube.com/watch?v=mLp_rSBzteI</url></story><parent_chain></parent_chain><comment><author>nsajko</author><text>Invented 1864!
&lt;a href=&quot;http://en.wikipedia.org/wiki/Schlieren_photography&quot; rel=&quot;nofollow&quot;&gt;http:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Schlieren_photography&lt;/a&gt;&lt;p&gt;And there&amp;#x27;s a variant in color:
&lt;a href=&quot;https://www.youtube.com/watch?v=0g4UBeaG5fs&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;www.youtube.com&amp;#x2F;watch?v=0g4UBeaG5fs&lt;/a&gt;</text></comment>