On Monday, August 21, 2017, a total solar eclipse will be visible in totality within a band across the entire contiguous United States, and will only be visible in other countries as a partial eclipse. Solar eclipses can be very dangerous if they’re directly observed without any eye protection.
The ONLY safe time to look at an eclipse with the naked eye is during the TOTAL phase of a TOTAL eclipse. And even then, you must ALWAYS use eye protection any time any piece of the Sun’s bright disk is visible. It is NEVER safe to look at the partial phases of an eclipse without proper protection. Looking directly at the sun or its eclipse can actually burn holes in the retinas and cause permanent blind spots in your vision.
Seeing a total eclipse can in fact be done quite safely with proper eye protection. But if you choose to stare at the Sun, outside of the window of time that it is being covered completely by the moon, and/or without proper equipment and techniques to ensure your safety, then you will likely do damage to your eyes. So don’t do that.
The only way to safely look at the Sun when it is not in total eclipse is through approved filter material designed and marketed expressly for direct solar viewing. If you do not have such material (which is inexpensive and readily available, through astronomical supply houses), then the only safe way for you to observe the partial phases of the eclipse is by using indirect methods such as pinhole projection, optical projection or using a sun funnel.
Directions for a pinhole projection box are found here:https://www.timeanddate.com/eclipse/box-pinhole-projector.html
Directions for an optical projection system are here:https://www.exploratorium.edu/eclipse-archive/optical2.html
Directions for a sun funnel device are here: https://www.youtube.com/watch?v=Y7o2vs62Cf4