  Traditional methods for measuring the speed of light in dispersive media have
been based on the detection of interference between light waves emitted from
the same source. In the present study the elapsed times for single photons to
move from a laser to a photomultiplier tube are measured electronically.
Time-correlated single photon counting detection produces a characteristic
instrument response which has the same shape independent of both the path
length the light travels and the nature of the transparent media through which
it passes. This allows for an accurate calibration of the chronograph by
observing shifts in the location of the instrument response for different
distances traveled by the light. Measurement of the corresponding shift which
occurs when light moves the same distance through air and water then enables an
accurate determination of the ratio of the photon velocities in these two
media. Three different wavelengths of light have been used. In two cases good
agreement is found between the present measured light speeds and those which
can be inferred from existing refractive index measurements in water. The
shortest wavelength studied is too far in the uv to obtain a reliable estimate
on the same basis, and so the ng value (1.463) measured in the present work
awaits independent confirmation. A theoretical discussion of the present
results is undertaken with reference to Newton's original corpuscular theory of
light. It is argued that his failure to predict that light travels more slowly
in water than in air arose from the inadequacy of his mechanical theory rather
than his assumptions about the elementary composition of light.
