It's false, you're not supposed to do that, however there was belief that it would be helpful to measure the distance to an atomic bomb detonation but it would be as helpful as taking advice from a turtle with the name of Bert. Supposedly it was going to be state issued advice like "Duck & Cover" but was never used encase it accidentally encouraged people to stand out in the open and gather fallout/ blind themselves by looking at the fireball. It's believed to be the origin of the saying "If it's bigger than your thumb you ought to run".
