The rainbow is such a remarkable phenomenon of nature, and its cause has always been so carefully sought after by good minds, yet so little understood, that I could not choose anything better to show you how, by means of the method I am using, we can arrive at knowledge not possessed by any of those whose writings we have. First, taking into consideration that this arc can appear not only in the sky but also in the air near us whenever there are drops of water in it that are illuminated by the Sun – as we can observe in certain fountains – it was easy for me to judge that it came merely from the way in which rays of light act against those drops, and from there tend toward our eyes. Then, knowing that these drops are round, as we have demonstrated above, and seeing that their size does not affect the appearance of the arc, I decided to make a very large [drop] so as to be able to examine it better.