He co-invented the electron microscope

He developed the ultramicroscope capable of studying objects below the wavelength of light

He reduced spherical aberration (or the "chromatic effect") by showing that several weak lenses used together at certain distances provided good magnification without blurring the image

He noticed pores or cells in a sliver of cork when he looked through a microscope. 

He was credited with  the invention the first wearable eyeglasses.