Although Michelson claims to have spent two months working with the apparatus it is curious that his first recorded set of measurements are with electric light at night. He then abandons this method in favour of natural light after observing that ``the image was no more distinct at sunset and the [electric] light was not steady''56. This suggests that some monitoring of the data occurred. He describes checking for other sources of error and making changes to his plan as he goes.
Had Michelson access to today's computational resources, it is likely that he would have at least monitored the speed determinations as they came in each day. Figure 10 is a plot of the recorded values for the speed of light in air versus the day of collection. Because so many values were recorded as identical, the plotted values have uniform random noise in the range from -4 to 4 added; this has the desired visual effect of spreading the points out in the plot.
There is an apparent decreasing relationship that is only stronger if the three outlying values are ignored. The noticeable exceptions to this relationship appear to be the values obtained on the last three days. Checking with the data we see that on the third last day Michelson inverted the rotating mirror R. After two days in this position, he inverted it again to get the original position. Arguably, these changes affected the process and prior to that time the study process seemed to be drifting downwards. Michelson does not seem to have noticed this.
