He observed that the
zinc plate lost its charge when it was illuminated by ultraviolet light Further, the uncharged zinc plate became positively charged when it was
irradiated by ultraviolet light Positive charge on a  positively charged
zinc plate was found to be further enhanced when it was illuminated by
ultraviolet light From these observations he concluded that negatively
charged particles were emitted from the zinc plate under the action of
ultraviolet light