After several introductory observations on the refractive condition of the human eye in infancy, in the prime of life, and in old age; and also in some tribes of artizans, in shepherds, and in mariners, with notices of the focal adjustment in the eyes of oxen, common fowls, and in fishes, as the cod and haddock, the author examined the different theories which have been advanced to account for this adapting power, and expressed his strong opinion that the external muscular apparatus was the principal and initiating agent.