At points commanding the mouth of the canyon, men were stationed to signal if the enemy approached from the west. The sentinels were provided with bits of mirrors to flash warnings. When General Miles wrote of his campaign against the Apaches he assumed that we knew nothing of the heliograph.
He was mistaken.
My people did not know the Morse code, but they had learned that mirror flashes warned soldiers and ranchers of our movements; and they, too, made use of the device. I’ve often wondered why so much has been written about smoke signals. As far as I know they were not much used.
James Kaywaykla. From In the Days of Victorio: Recollections of a Warm Springs Apache by Eve Ball (James Kaywaykla, narrator)
In the "Talking Mirrors" lesson created by the Center for Image Processing in Education for the Consortium for Optics and Imaging Education, students learn about the law of reflection by studying the heliograph system established by General Miles in 1886 to help track down Geronimo.
Created in 2004, the lesson is offered by Science Approach in support of Arizona's centennial celebration on 14 February 2012.
You may access the lesson by visiting http://science-approach.com and clicking the Register! link in the main menu. Once you have registered at the site, login and click the "Enter" button under the Materials and Subscriptions heading at the top-right of the screen. After completing a brief profile, you'll be directed to the My Labs & Resources page. "Talking Mirrors" is part of the Guiding Light materials included under the Free Software and Materials link.