Shown here is a very large foil-filled flashbulb, about the largest I've seen. I have heard they were used by the military during WWII, supposedly to 'take pictures from spy airplanes at night' (not sure how that would work). I've also read about them being used to take pictures of large, dark caves. There are smaller sizes of these bulbs too (see related links below). I believe the foil is magnesium foil. The full name of this bulb is "G.E. MAZDA Photoflash Lamp No. 75".