Practical Treatise on the use of the microscope. J. Quekett, 1855. Bailiere Publishers, London. p29.
Withering's original description was in: A Botanical Arrangement of all the vegetables naturally growing in Great Britain, 2 volumes (1776). A description of his life can be found here.
William Withering's Botanical Microscope, 1776
For a study of the Withering microscope, especially one that Charles Darwin used on the Beagle Voyage see this link.