Application of lasers can be divided into two broad categories: (1) commercial, industrial, military, and medical uses, and (2) scientific research. In the first case, lasers are used to do something that has been done in another way up to now (but not as well). Sometimes a laser solves a particular problem. For example, one of the first applications was in eye surgery, for “welding” a detached retina. The laser is particularly useful here because laser light can penetrate transparent objects such as the eye’s lens (Figure 19), eliminating the need to make a cut into the eye. [Illustration: Figure 19 _Diagram of human eye showing laser beam focused on retina._] Cornea Lens Optic Nerve Beam angle Fovea centralis Iris Image Retina Surgeons have long wanted a better technique for treating extremely small areas of tissue. A laser beam, focused into a small spot, performs perfectly as a lilliputian surgical knife. An additional advantage is that the beam, being of such high intensity, can also sterilize or cauterize tissue as it cuts. The narrowness of the laser beam has made it ideal for applications requiring accurate alignment. Perhaps the ultimate here is the 2-mile-long linear accelerator built by Stanford University for the United States Atomic Energy Commission. “Arrow-straight” would not have been nearly good enough to assure expected performance. A laser beam was the only technique that could accomplish the incredible task of keeping the ⅞ inch bore of the accelerator straight along its 2-mile length. A remote monitoring system, based on the same laser beam, tells operators when a section of the accelerator has shifted out of line (due for example to small earth movements) by more than about ¹/₃₂ inch—and identifies the section.[14] Figure 20 shows the 2-mile-long “klystron gallery” that generates the power for kicking the high-energy particles down the tube. The gallery parallels the accelerator housing and lies 25 feet beneath it (Figure 21). The large tube houses the optical alignment system and supports the smaller accelerator tube above. Target patterns dropped into the large tube at selected points produce an interference pattern at the far end of the tube similar to the one in Figure 13. Precise alignment of the tube is achieved by aiming the laser at the center dot of the pattern. Then the section that is out of line is physically moved until the dot appears in the proper place at the other end of the tube. It is the extreme coherence of the laser beam that makes this technique possible.