Why could William Harvey not directly observe capillaries?

William Harvey could not directly observe capillaries because the microscope had not yet been sufficiently developed to reveal these tiny vessels during his lifetime.12

Although Harvey correctly hypothesized that a system of vessels must link the arteries and veins to complete the circulatory system, he lacked the technology to confirm their existence. The connection was only observed later in the 17th century by Marcello Malpighi, who used a primitive microscope to identify the network of vessels in a frog's lung.12