ABSTRACT The limit of the resolvability of lines, or how small a space can exist between lines and still admit of their being separated under the microscope, appears to be an undecided point. Professor Queckett (‘Treatise on the Microscope,’ 3d ed., p. 238, 1855) asserts that “no achromatic has yet been made capable of separating lines closer together than the of an inch.” In the same work, p. 245, it is stated that Mr. Ross found it impossible to ascertain the position of a line nearer than the of an inch. We find also on p. 512, that Mr. De la Rue, in his extended examination of Nobert’s test-plates, was unable to resolve any lines closer than the of an inch. In Professor Carpenter’s work (‘The Microscope,’ 2d ed., p. 189, 1859) this sentence occurs: “The well-defined lines on Nobert’s test-plates have not yet been resolved -when they have approximated more closely than the of an inch.’’