two slits whose distance apart must be changed. This can be done by mounting the mirror a and the mirror b on a right- and left-handed screw. On turning the screw the two mirrors would move in opposite directions through equal distances, leaving everything else unchanged. Such an instrument is represented in Fig. 102. The light falls from below upon the two mirrors a and b, which are mounted on carriages which can be moved in opposite directions by the right- and left-handed screw.
Fig. 103 represents an actual instrument which was used in making laboratory experiments to test the method. The artificial double stars, or star discs, were pinholes made in a sheet of platinum. These holes were as small as it was possible to make them, of such a diameter as to test the resolution of the telescope, with a bright source of light behind them. The left-hand figure represents the double slit. It is mounted on a right- and left-handed screw and can be operated by the observer. The slits can thus be moved by a measurable quantity, and their distance apart when the fringes disappear can be determined.
After making a series of such experiments in the laboratory, I was invited to spend a few weeks at the Lick Observatory at Mount Hamilton to test the method on Jupiter's satellites. These satellites have angular magnitudes of something like one second of arc, so that they should be measurable by this
