When viewed through a magnifying lens against a zinc sulphide screen, radium bromide can be observed surrounded by tiny flashes of light. This is the effect caused by the individual impacts of alpha particles. William Crookes discovered this phenomomen by mistake in 1903; at that time radium bromide was the most expensive substance on earth, and Crookes accidentally spilled the small amount he had obtained onto the screen he was using to observe their greenish glow. On attempting to recover all the radium particles with the help of his microscope, Crookes discovered the phenomenon of these flashes and wrote "On bringing the radium nearer the screen, the scintillations become more numerous and brighter, until when close together the flashes follow each other so quickly that the surface looks like a turbulent, luminous sea."