Complex sentence: Morley experiment, which aimed to measure differences in the relative speed of light due to the Earth's motion through the hypothetical luminiferous aether, which was then considered to be the medium of propagation of electromagnetic waves such as light.
Split into simpler sentences: Morley experiment, which aimed to measure differences in the relative speed of light due to the Earth's motion through the hypothetical luminiferous aether. The aether was then considered to be the medium of propagation of electromagnetic waves such as light.