An incident laser beam encounters a beam splitter. The two subsequent beams bounce off of mirrors and then rejoin. Interference results from the path length difference of the two beams. There is also a compensation plate which acts to give both beams the same amount of travel through glass. The two beams will appear on the screen along with an unfortunate secondary reflection. Adjust the bottom mirror until two of the beams overlap and an interference pattern is seen. The use of diverging lenses either on the incident beam or on the resultant two beams will make the pattern more visible. The upper mirror can be adjusted with the Vernier screw to show the effects of changing the length of one of the path lengths.