The Laws of Blessings which apply during the Rainy Period (19)

1) One may make the blessing when one sees a complete semicircle of rainbow, but when one sees only a small part of it, it is unclear whether one may make the blessing (Biur Halacha 229). Some say that in this case one may not pronounce the Name and Kingship of Hashem. (Teshuvos Vehanhagos 3:76)

2) One may make the blessing on a rainbow even when one sees it through a glass window. (AA”Ch 115)