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The B-curve even for 10 will, therefore, deviate but
London, October 1917,
Research Dept., Adam Hilger, Ltd.
VI. Interferometers for the Experimental Study of Optical
1. Description of the Interferometers.
1. Description of the Interferometers.
HESE instruments in their simplest form resemble the
Optical elements or combinations suitable for examination
The Prism Interferometer.
The prism interferometer is shown in diagram (fig. 1) as
The light used must consist of monochromatic rays. Such
a light may be obtained from a Cooper-Hewett MercuryVapour Lamp, combined with a suitable filter.
The light from the source is reflected by the adjustable mirror A through the condensing-lens B, by means of which it is condensed on the aperture of the diaphragm C.
The diverging beam of light is collimated by a lens D, and falls as a parallel beam on a plane parallel plate K, the second surface of which is silvered lightly so that a part of the
Diagram of Prism Interferometer.
light is transmitted and part reflected. The major part should be reflected. One part passes through the prism L in the same way as in actual use, and being reflected by the mirror F passes back through the prism to the plate K. The other part of the light is reflected to the mirror G and back again to the plate K. Here the separated beams recombine, and passing through the lens E each forms on the eye placed somewhat beyond the aperture in the diaphragm P an image of the hole in the diaphragm C. One of the reflecting mirrors in its mount is shown in fig. 2.
When the mirrors are adjusted, interference-bands are seen which form a contour map to a scale of wave-lengths of the extent to which passage of the beam twice through the prism has distorted the wave-front. This distortion can be Fig. 2.
Mirror in Mount.
corrected by removing from each point of one prism face, by local polishing, an amount of glass proportional to the distortion of the wave-front at that point; hence it follows that the bands also form a contour map of the glass requiring to be removed in order to make the optical performance of the prism perfect. Fig. 3.
Fig. 3 represents in diagram a typical map, where Q represents the highest point of a "hill." The procedure in such a case is to mark out the contour lines on the surface of the prism with a paint-brush dipped in rouge, and then to
polish first on the region Q, subsequently extending the area of polishing first partly, then wholly, to the next contour line; and so on. The marking out of the prism surface can be done while observing the bands.
It should be noted that variations in the contour lines are obtained by a tilt of the plane of reference. Thus a slight adjustment of mirror F (fig. 1) might change a contour map
from that shown in fig. 4 to that shown in fig. 5. The form of surface is in each case the same (see the sectional diagrams at the top of the figures), but correction can be carried out according to whichever plane of reference is the most favourable from the point of view of the operator. In order to find whether Q (fig. 3) is a hill or a valley, the cast-iron table M (fig. 1) can be bent with the fingers so as to tilt the mirror F in such a way as to lengthen the ray-path. If the contour line at Qexpands to enclose a larger area, a hill is indicated, and vice versa. Although the words "hill" and "valley" are convenient to use, it must not be supposed that the imperfections necessarily result from want of flatness either of one or of both surfaces of the prism. The contour map gives the total effect on the wave-front produced by double passage through the prism, and shows in wave-lengths the departure from planeness of the resulting wave-surface.
Diagram of Lens Interferometer.
In the lens interferometer all parts are left as in the prism arrangement except that the mirror F is removed and replaced