The Laws of Blessings which apply during the Rainy Period (17)

1) One who sees a rainbow should not tell his friend as it is a bad sign (as it shows that really a flood should have come to the world at that time), and it says “One who brings out bad speech is a fool!”. (Shulchan Aruch and Mishne Berurah 229)

2) One may not look a lot at a rainbow. Instead, one should see it and make the blessing. One who looks a lot at it is punished in that his eyes go dark (his sight becomes weak). (ibid)