Treatise on Light

experience, since the angle G E C would be very noticeable, and about 33 degrees. For according to our calculation, which is in the Treatise on the causes of the phenomena of Saturn, the distance B A between the Earth and the Sun is about twelve thousand terrestrial diameters, and consequently four hundred times greater than B C, the distance of the Moon, which is 30 diameters. Therefore the angle E C B will be nearly four hundred times greater than B A E, which is five minutes—namely, the path that the Earth travels in two hours in its orbit; and thus the angle B C E is almost 33 degrees; and likewise the angle C E G, which exceeds it by five minutes.

But it must be noted that the speed of light in this reasoning was posited such that it requires one hour of time to travel the path from here to the Moon. But if one supposes that it requires only one minute of time for that, then it is manifest that the angle C E G will only be 33 minutes; and if it requires only ten seconds of time, this angle will not be [even] six minutes. And then it is not easy to notice it in eclipse observations, nor consequently permitted to conclude anything from it regarding the instantaneous movement of light.

It is true that this is to suppose a strange speed which would be a hundred thousand times greater than that of Sound. For Sound, according to what I have observed, travels about 180 ToisesA toise is an old French unit of length, approximately 1.949 meters or 6.4 feet. This measurement suggests a speed of sound around 350 meters per second. in the time of one Second or a heartbeat. But this supposition should not seem to have anything impossible [about it]; because it is not a matter of the transport of a body with such speed, but of a successive movement that passes from one [particle] to another. I have therefore found no difficulty, in meditating on these things, in supposing that the emanation of light occurred over time, seeing that by this all its phenomena could be explained, and that by following the contrary opinion everything was incomprehensible. For it has always seemed to me, and to many others with me, that even Mr. Des CartesRené Descartes (1596–1650), the famous philosopher and scientist. He believed light was a pressure transmitted instantaneously through a "plenum" (space full of matter), an idea Huygens is here challenging., whose goal was to treat [things] intelligi-