Science: Precision and Stability

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Dr. Louis Caryl Graton, professor of mining geology at Harvard, last week issued the following statement:

''A new microscope developed at Harvard incorporates such novel features of design as to permit a new step forward in the scientific investigation of the extremely minute. Somewhat paradoxically, the instrument itself is of great size and massive proportions, weighing about one ton, in order to insure utmost precision and stability."

Blue-eyed, white-haired, courtly Dr. Graton was aware that news of his microscope had created something of a furor among authorities in optical science, because it was upsetting to optical theory. Late in the 19th Century, Ernst Abbe, a great optical physicist who worked for Carl Zeiss of Jena, laid down a theory fixing the upper limit of useful magnification at 1,500 diameters. Microscopists have sneaked considerably beyond the Abbe limit with an instrument called the ultramicroscope, in which very small particles are strongly illuminated from the side. This enables them to be counted, if they are widely enough separated, although their shape is not disclosed. Also the use of ultraviolet light, too short to be seen, makes it possible to photograph submicroscopic objects.

But for straightforward magnification in visible light, said Professor Graton, Abbe's 1,500-diameter limit "has ever since been generally accepted as reliable." The Harvard microscope, however, is reported to have a useful range up to 6,000 diameters. In specimens of ore it can spot flecks of gold so small that 40,000,000,000 of them are worth only one cent.

To reduce vibration to an absolute minimum, the one-ton instrument is housed in an underground chamber and set in 15 tons of concrete. The parts were machined with the utmost possible precision by a Cambridge instrument maker. The lenses are ordinary high-grade lenses such as are manufactured by Zeiss or Bausch & Lomb. The focus, however, is geared down 100 times finer than in any stock microscope. To change the focus by one millimetre would require 25 minutes of hand turning on the focusing knob. Such tedious labor is avoided by driving the focusing adjustment with an electric motor. When the motor is running the focusing position is indicated on a calibrated dial whereon one unit represents 2½ millionths of an inch.

Dr. Graton achieved his high magnification on mineral specimens polished to mirror smoothness in a grinding machine of his own devising. He believes the instrument would be equally useful for biological work. By no means a publicity seeker, he had the microscope in operation for a long time before first publishing a description of it last month.

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