According to Maxwell's four equations, we can solve the propagation of electromagnetic waves in vacuum, and the combination of some quantities in the equation represents the speed of electromagnetic waves. As a result, Maxwell got this "certain quantity" as follows: the dielectric constant in vacuum is multiplied by the magnetic permeability of vacuum, then the root sign is opened, and then the reciprocal is taken. Through the measurement of these quantities by others, the measured values are brought into this item, and the specific values of electromagnetic waves in vacuum are obtained, which are exactly the same as the speed of light. So Maxwell thinks that light is electromagnetic wave.