OPTICS, 135 
is the aperture CD (fig. 96), we may consider all the particles of ether at 
the opening as being in similar phases of vibration. Each one propagates 
its vibrations in every direction beyond the screen, and the intensity of 
illumination at any point, s, on the other side of the screen, depends upon 
the result produced by the interference of rays passing through different. 
points of the aperture CD and meeting in s. Hence it follows that rays 
transmitted at right angles to CD will strengthen each other, and conse- 
quently give rise to a bright stripe or spot in the centre, while on each side 
or around this centre, dark and light stripes or circles must alternate. The 
phenomena exhibited by a parallelogram-shaped aperture (fig. 94) are 
produced in a similar manner. Thus, the parallelogram abcd (fig. 97) 
forms a part of a vertical slit, and therefore presents a succession of 
horizontal spectra, while the edges, ab, cd, form part of an obliquely trans- 
verse slit, and produce spectra following each other in the direction of the 
line /m, perpendicular to the edges ab, ed. 
If two or more diffracting apertures of equal size and shape stand 
near each other, the same figure as that produced by a single one will be 
seen, only intersected by many black stripes. These, according to 
Fraunhofer’s terminology, convert spectra of the first order into those of 
the second. Through two parallelogram-shaped apertures we have the 
appearance represented in fig. 98, and through three circular ones that in 
fig. 99. 7 
Peculiar phenomena, first discovered by Farapay, are observed whenever: 
we look through a telescope, before which is placed a fine wire grating, at 
a line of light parallel to the intervals of the grating. If white light be 
employed, smaller colored spectra are produced, intermixed with black 
interspaces. If, instead of the grating, a fine gauze be used, the spectra, 
radiating from a centre, present a highly beautiful appearance. 
Colors of thin plates.—If a glass lens of great radius of curvature be 
pressed upon a flat plate of glass, a series of concentric colored rings will 
be observable around the central dark point of contact. These are also 
observed in the case of thin films of oil, metallic oxides, &c.; in fact, the 
illustrations of the phenomenon may be varied infinitely. They are all 
explicable on the undulatory theory, by the interference produced by the 
reflection of rays from the upper and under surfaces of the thin plate, this 
consisting, in the first-mentioned experiment, of the film of air interposed 
between the two plates of glass. The different degrees of interference 
between the two sets of reflected rays, produce the various shades of color 
and light. Light transmitted through thin plates also exhibits the colored 
rings or bands, these being complementary to colors of reflection. Thus, 
if in the first-mentioned experiment, the colors reflected are black, blue, 
white, yellow, red, those transmitted will be in succession, white, yellowish 
red, black, violet. blue, &c. Below and above a certain thickness of the: 
plate, these colors cease to be visible, this thickness varying with different 
media. Thus for air the minimum is half a millionth of an inch, the 
maximum seventy-two millionths ; for water, three eighths of a millionth, 
309 
