The 1.509 is the refractive index (or change in the speed of light) of the most common glass used for prisms.
(The 1.509; is; the refractive index (or change in the speed of light) of the most common glass) (the most common glass; used; for prisms)
The 1.509 the most common glass
is used
the refractive index (or change in the speed of light) of the most common glass
for prisms