How to Watch a Partial Solar Eclipse Safely
Pinhole projection. The simplest safe way to view a partial solar eclipse is to watch the Sun's image projected onto a piece of paper. Poke a small hole in an index card with a pencil point, face it toward the Sun, and hold a second card three or four feet behind it in its shadow. The hole will project a small image of the Sun's disk onto the lower card. This image will go through all the phases of the eclipse, just as the real Sun does. Experiment with different size holes. A large hole makes the image bright but fuzzy; a small hole makes it dim but sharp.
A much better way to do pinhole projection can be arranged at a window indoors. Find a room with a Sun-facing window, turn out any lights, and pull the shades. Arrange for sunlight to enter through a small hole punched in a card near the top of the window. Set up a white piece of paper across the room to catch the Sun's image. Again, experiment with different size holes to get the best, sharpest view. (Of course, don't look through the hole directly at the Sun! Look only at the spot of light that falls on the paper.)
If the Sun is too high in the sky for this, you can direct its image horizontally into the room by setting up a small, high-quality mirror on the sill of an open window. Hold the mirror in place with modeling clay. Tape your card with the hole right onto the mirror.
Even at its best, pinhole projection gives only a small image. The throw distance in feet, divided by 9, gives the image diameter in inches. Pretty small!