and the Luminiferous Hther. 123 
lit, a small hole made in a screen placed before it served asa 
point of light; and the plate 6, which was adjustable in two 
planes, was moved about till the two images of the point of 
light, which were reflected by the mirrors, coincided. Then a 
sodium flame placed at a produced at once the interference 
bands. These could then be altered in width, position, or 
direction, by a slight movement of the plate 6, and when the 
were of convenient width and of maximum sharpness, the 
= 
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q 
2 
sodium flame was removed and the lamp again substituted. 
he screw m was then slowly turned till the bands reappeared. 
They were then of course colored, except the central band, 
which was nearly black. The observing telescope had to be 
focussed on the surface of the mirror d, where the fringes were 
most distinet. The whole apparatus, including the lamp and 
the telescope, was movable about a vertical axis. 
It will be observed that this apparatus can very easily be 
