Traditional flat dive masks trick your brain with a powerful optical illusion. Underwater refraction makes you believe that your field-of-view is much greater than it really is. Physiological feedback confirms that your eyes scan far left, right, up, and down. But if you reach out your arms to match your eyeballs' pointing angle...  you cannot see your arms! Only when your arms arc inward, closer to the axis where your nose points, can you see your arms.