Microscope, Telescope, Interferometer 41
as the bed of the instrument. One end of this bed has fastened to it a heavy metal plate i7, which carries the three glass plates A, Z), and B, The plate A is held in a metal frame which is rigidly fastened to the plate H. The frame which holds B can be turned slightly about a vertical axis to allow of adjusting B so that it is parallel to A.
The mirror D is held by springs against three adjusting screws which are set in a vertical plate attached to the end of the plate H. Both C and D are silvered on their front faces. The frame which holds the mirror C is firmly mounted on a metal slide which can be moved by the screw S along the ways EF. One very essential feature of the apparatus is that these ways shall be so true that the mirror C shall remain parallel to itself as it is moved along. The accuracy of the ways must be so great that the greatest angle through which the mirror C turns in passing along them is less than one second of an arc.
This accuracy cannot be attained by the instrument maker, but the final grinding must be done by the investigator himself.
To adjust the instrument so that fringes are formed, a small object like a pin is held between the source and the plate A. Two images of this pin will be seen by an observer at O—one formed by the light which is reflected from C, and the other by that reflected from Z). The fringes in
42
Light Waves and Their Uses
monochromatic light will appear when these two images have been made to coincide with the help of the adjusting screws ss. The fringes in white light appear only when the lengths of the two paths AD and AC are the same. The
FIG. 40
width and the position of the fringes in the field of view can be varied by slightly moving the adjusting screws. We shall have occasion to discuss this particular form of interferometer in a subsequent lecture.
summary
1. The objection to the wave theory of light, that light moves in straight lines while sound waves can bend around an obstacle, is shown to be groundless, since we have seen that if the sound waves are sufficiently short they cast a sound shadow, while by devices which take into account the