What did Robert hooke look at through his microscope?

He did not. The Microscope was invented by Leuwenhoek, a Dutch Lens Grinder in the mid l7Th century in Delftt, a humorous poem was written about it. the original model used compound lenses and had a high magnification of 300 X. Robert Hooke may have made improvements on microscopes, as a hobby manufacturer implied with a kit of Hooke"s microscope circa l962, but he did not Invent the instrument.