Put things in their place

Use the microscope, mercury light source, and monochromator of the Fraunhofer diffraction experiment, but instead of a slit employ a razor blade for the diffracting object. Carefully determine the focal plane of the microscope, and measure the distance $a+b$ from that point to the source - the exit slit of the monochromator. Move the edge back to about 30 cm from the source, and locate it so that the pattern from it falls squarely on the microscope. Make a note of its distance, $a$, from the source. For the green mercury line, with a wavelength of 5461 Å, observe the pattern in the microscope. For convenience in interpreting the results, the edge should be oriented so that the shadow zone corresponds to small $x$, and the bright zone with fringes corresponds to large $x$. These fringes will have much lower contrast than the Fraunhofer fringes.