real, though in all ordinary cases it is masked by the action of
interference. This inflection of the light receives the name of
_Diffraction_.

To study the phenomena of diffraction it is necessary that our source
of light should be a physical point, or a fine line; for when a
luminous surface is employed, the waves issuing from different points
of the surface obscure and neutralize each other. A _point_ of light
of high intensity is obtained by admitting the parallel rays of the
sun through an aperture in a window-shutter, and concentrating the
beam by a lens of short focus. The small solar image at the focus
constitutes a suitable point of light. The image of the sun formed on
the convex surface of a glass bead, or of a watch-glass blackened
within, though less intense, will also answer. An intense _line_ of
light is obtained by admitting the sunlight through a slit and sending
it through a strong cylindrical lens. The slice of light is contracted
to a physical line at the focus of the lens. A glass tube blackened
within and placed in the light, reflects from its surface a luminous
line which, though less intense, also answers the purpose.

In the experiment now to be described a vertical slit of variable
width is placed in front of the electric lamp, and this slit is looked
at from a distance through another vertical slit, also of variable
aperture, and held in the hand.

The light of the lamp being, in the first place, rendered
monochromatic by placing a pure red glass in front of the slit, when
the eye is placed in the straight line drawn through both slits an
extraordinary appearance (shown in fig. 15) is observed. Firstly, the
slit in front of the lamp is seen as a vivid rectangle of light; but