While silver bromide crystals are wonderfully sensitive to light, they do not respond to all wavelengths of light equally. This accounts for a paradox in black and white photography: colors, or different wavelengths within the visible part of the electromagnetic spectrum, must be considered when making black and white black and white pictures. Different types of black and white film react to color differently, as the three black and white pictures at right indicate. Unless silver bromide crystals are specially treated, they respond only to the shorter wavelengths of light, from ultraviolet through blue-green. Black and white photography got past this stumbling block back in 1873, when H. W. Vogel added a dye that extended the response to green and yellow wavelengths. By a mechanism still not completely understood, the dye absorbs these slightly longer wavelengths and transfers their energy to the silver bromide crystals. This sort of emulsion is known as orthochromatic, and is used today mostly to make black and white photographic copies. Other dyes now enable black and white film to record all the colors seen by the human eye; this panchromatic black and white film is the type almost - universally used in ordinary black and white photography. However, it does not respond to colors evenly. It is more sensitive to short wavelengths (bluish colors) than to long wavelengths (reddish colors). Unless this imbalance is compensated by filters (pages 176 to 178), blue sky tends to come out very bright, red apples are dark and even green leaves seem darker than they ought to. Special dyes have also been devised to make black and white films respond to invisible infrared wavelengths in addition to all the visible colors-with results (far right) that are eerie but often beautiful. In a black and white photograph made on orthochromat black and white film, some of the fruits and vegetables ac; come out looking darker than they human eye, because the black and white film responds to shorter wavelengths-toward the the spectrum (above)-and is incense, reddish colors. The apple, orange and re: pepper (upper right) and the red onion (lower left) all appear unnaturally dim. Infrared black and white film records visible colors as well as some longer wavelengths that are not visible. Although most natural objects strongly reflect infrared rays, there is no consistent relationship between the color of an object and the amount of infrared rays that are reflected. In the black and white picture above, only the avocado and the mushrooms do not reflect infrared strongly; thus they appear darker than the other objects.