You actually can't see the decay of radium directly. Radium decay emits alpha and beta radiation. Alpha radiation is stopped by the skin; beta radiation is slightly more penetrating and might occasionally cause a flash in the eye. read more
Human eyes are sensitive to a limited range of radiation wavelength, which is called the visible spectrum, and the wavelength(es) of nuclear radiation is much shorter than that of visible spectrum(meaning a higher frequency and carrying more energy and can be more harmful to human body). read more