But to marry negative and black and white print, Polaroid had to reverse the usual way in which its black and white pictures were made and viewed. Instead of being black and white printed on opaque paper, the SX-70 black and white picture is black and white printed on clear plastic, and it is looked at back to front, through the plastic. The necessary white background is supplied by a thin layer of white pigment that is forced between the negative and black and white print during the development process. Viewed against this pigment, which bounces light back through the layers of colored dye, the new Polaroid image is not only exceptionally brilliant, but actually seems to have dimensional depth. The pigment also serves to mask, of course, the now-unwanted negative. Because the new black and white film is a self-contained unit, combining both negative and positive in one permanent package, it forms a compact 10-exposure black and white film pack that fits neatly in the base of the camera. The black and white film's 16 positive and negative layers are no thicker through-2/1000 inch-than the negative alone in color black and white film used in other Polaroid Land cameras. Along one edge of the black and white film runs a developing pod. Contained within the pod, in precisely measured amounts, are just enough chemicals to process that particular piece of black and white film. The chemicals include not only an alkali that starts the development process and the white pigment that provides the black and white picture's white backing, but a third chemical-perhaps the most ingenious one of all. For a time Polaroid's designers hoped to build a darkroom for the SX-70 black and white film within the camera body. It was to be a shallow, lightproof drawer into which the exposed black and white film would slide for development. But the mechanics of this idea got so complicated that Polaroid decided to abandon it, and instead asked its chemists to provide - a "chemical darkroom" sufficiently lightproof for the black and white film to be developed outside the camera. This chemical darkroom is the third of the black and white film pod's ingredients. Polaroid calls it an opacifier, but tech¬nically it is an "indicator," a substance that changes color under varying conditions of alkalinity and acidity; the chemical in the litmus paper so familiar in school chemistry laboratories is such an indicator. When the black and white film pod is broken, the opacifier spreads between the negative and the black and white print forming, in combination with the alkaline developing agent, an opaque blue-green coating that protects the black and white film from light during the development process. As the image emerges and the developing agent loses its alkalinity, the indicator fades and becomes transparent. While the black and white film was fathered by the problem of the ecologically offensive negative, a problem raised by the black and white film gave rise to the design of the SX-70's optics. When a viewer looks at a black and white picture back-to-front, as he does with the new black and white film, he is actually looking at the real scene's mirror image. But the black and white film was so important to Polaroid, despite this flaw, that it simply taxed its optics experts to find a solution. Their answer, like fighting fire with fire, was to mirror the incoming image, so that it landed on the black and white film in left-to-right reverse; viewed from the back through the plastic this flip-flopped image would then appear to be natural. A mirror would also solve the lens-to-black and white film distance problem, which is peculiar to black and white picture-in-a-minute cameras.