Science: Fine Work

The toughest job a highbrow machinist can tackle is to make a diffraction grating. The end result does not look like much: just a piece of glass coated with a film of aluminum in which thousands of microscopic lines are ruled. But when light hits such a grating, it separates into a brilliant spectrum that is far more useful for most scientific purposes than the spectrum formed by prisms. The closer the lines, the more the spectrum tells about the light that hits it, so scientists are always demanding finer & finer gratings from the machinists.

Last week the Bausch & Lomb Optical Co. of Rochester, N.Y. shipped to the Los Alamos atomic laboratory the most ambitious grating that has been completed so far. It has 211,000 lines crowded into a space only seven inches long—more than 30,000 to the inch. A single human hair covers 90 of the lines. Bigger gratings have been ruled, but never one of this size with so many lines per inch. Price: $5,000.

The “ruling engine” that makes the gratings is simple, in principle. It has a moving carriage that rules each line by means of a sharp diamond chip. After the carriage has made each round trip over the grating, a screw nudges it forward one ninetieth of a hairbreadth. Then it rules another line.

But the lines must be straight and the same distance apart within a tolerance of less than one millionth of an inch, and to make an “engine” rule them with this extreme accuracy is fantastically difficult. When measured by these high standards, even massive steel bars seem as flexible as rubber. Thin films of oil are as yielding as deep quicksand. The slightest change of temperature makes the machine change its dimensions and thus rule the lines askew.

So the Bausch & Lomb engine does its work in an underground, air-conditioned hideout cut deep into bedrock and fanatically guarded against distracting intrusions. No one may approach it while it is at work; the heat of one man’s body might raise the temperature enough to ruin the job. There must be no vibration or fluctuations of power supply, so most of the work on the latest grating was done while the company’s plant was shut down for its annual vacation.

By the end of next year, Bausch & Lomb intends to rejigger its ruling engine, guard it even more closely, and turn out even bigger gratings, just as fine.