"Direct the telescope upon the sun as if you were going to observe that body. Having focused and steadied it, expose a flat white sheet of paper about a foot from the concave lens; upon this will fall a circular image of the sun's disk, with all the spots that are on it arranged and disposed with exactly the same symmetry as in the sun. The more the paper is moved away from the tube, the larger this image will become, and the better the spots will be depicted. Thus they will be seen without damage to the eye, even the smallest of them - which, when observed through the telescope, can scarcely be perceived, and only with fatigue and injury to the eyes...I have demonstrated that the sunspots are neither stars nor permanent materials, and that they are not located at a distance from the sun but are produced and dissolved upon it in a manner not unlike that of clouds and vapors on the Earth."
-Galileo Galilei, Istoria e dimostrazioni intorno alle macchie solari e loro accidenti, Rome: Appresso Giacomo Mascardi, 1613