In an extensive series of observations conducted during the 1930s and '40s, Lowell Observatory astronomers Earl Slipher and James Edson took thousands of photographs of the cusp extensions through monochromatic filters
. They reported that these features appeared generally similar in red, yellow, and blue light, though they usually differed in length and brightness and often contained prominent reddish spots.
They were discovered at Mount Wilson Observatory by Frank Ross, a pioneer in the photography of the planets through monochromatic filters
. During a favorable eastern elongation of Venus in June and July of 1927, he obtained a series of photographs of the planet through the 60-inch and 100-inch reflectors in six regions of the visible spectrum and in infrared and ultraviolet light.