Flint Glass

Also called Crystal, or Lead Crystal, heavy and durable glass characterized by its brilliance, clarity, and highly refractive quality.  From Encyclopedia Britannica


"...Crown and flint glasses perform differently with varying wavelengths, and in practice the lens manufacturer uses these two glasses in such a manner that equal and opposite amounts of chromatic aberration are induced - this process of bringing two or more spectral lines together is called achromatization. Achromatic lenses have been designed to bring two wavelengths (normally red and blue) into precise coincident focus."  
From Infrared Photography 

  Songs referenced: 

Amber Waves