With respect to comprehension of the transmission of light through space, Maxwell's article on the light-bearing ether can arguably be considered the high point. The behavior of light as an electromagnetic wave had been accurately expressed in the set of equations that bear his name. That behavior could be accounted for by the existence of the ether, the essential properties of which were known from logic and experiment. True, the physical details of the ether had not been worked out, but there was reason to hope they would be, through time and experimentation. That was in the year 1878. Nine years later, an experiment by Michelson and Morley would trigger a process of retrenchment that would end, around the turn of the century, in the demise of the ether as a viable physical substance, and the consequent loss of the potential to understand why light behaves as it does.
At the time of the Michelson-Morley experiment, it was supposed, based on experimental evidence, that the ether is more or less stationary relative to the Sun. That being the case, the Earth's movement around the Sun is movement through the ocean of ether. The aim was to measure the speed of the Earth through the ether.
The experiment used an apparatus similar in principle to the one described by Maxwell in the previous article, though highly refined. The refinements were designed to produce a measurable shift of the interference pattern on the screen as the apparatus was moved in and out of alignment with the movement of the Earth through the ether. We won't get bogged down in a discussion of the details of the experiment; interested readers are encouraged to see this Wikipedia article.
We will discuss the physical phenomenon that the experiment was designed to detect, as it is key to all that follows in this section on the transmission of light through space. The concept is familiar enough, as we shall see with the help of this illustration.
The dot marked A represents some physical object that is stationary in the ether. The beam of light originates at the light source and travels through the ether toward A, at light speed c. The time required for the beam to travel from the source to A is the distance divided by the speed, D/c. Now consider the dot marked B, which represents a second object. Object B is moving through the ether toward the light source at some speed v. At the instant the beam of light is emitted from the source, objects A and B are side by side, at distance D from the source. Clearly, the light beam will reach object B before it reaches object A, because object B is moving to meet the beam while object A remains in its original position. The velocity of object B in relation to the light source is c + v. The light beam will reach object B at time D/(c + v), which is less than D/c.
The difference in travel times can be detected by the experimental apparatus. (See the sketch, below.) The apparatus is arranged so that the two outboard mirrors, and thus the two paths of light, are set at right angles to each other. As the entire apparatus moves through the ether, it can be oriented so that one of the paths is parallel to the motion of the apparatus while the other path is perpendicular. In that orientation, the mirror on the perpendicular path will act as Object A, and the mirror on the parallel will act as Object B. Because the path lengths are equal, there will be a small difference between the elapsed times to traverse the paths. (The elapsed times in the apparatus are somewhat different from the times calculated in the example, due to the motion of the apparatus through the ether, and the reversal of the beams at the mirrors.) That difference will cause a shift of the interference pattern on the detector.
The preceding describes how the experiment was expected to work. In the event, the observed shift of the interference pattern was much smaller than expected, so small as to be considered zero within the range of experimental error. The effect of the ether was not detected. This null result would have to be accommodated somehow.