The next Part of the _Essay_ (sect. 121-45) argues for a specific as well as a numerical difference between the original data of sight and the data of touch and locomotion. Sight and touch perceive nothing in common. Extension in its various relations differs in sight from extension in touch. Coloured extension, which alone is visible, is found to be different in kind from resistant extension, which alone is tangible. And if actually perceived or concrete extensions differ thus, the question is determined. For all extension with which man can be concerned must be concrete (sect. 23). Extension in the abstract is meaningless (sect. 124-25). What remains is to marshal the scattered evidence, and to guard the foregoing conclusions against objections. This is attempted in sections 128-46. ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ The enunciation of the summary generalisation, which forms the “New Theory of Vision” (sect. 147-8), may be taken as the Fifth and culminating Part of the _Essay_. ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ The closing sections (149-60), as I have said, are supplementary, and profess to determine the sort of extension—visible or tangible—with which Geometry is concerned. In concluding that it is tangible, he tries to picture the mental state of Idominians, or unbodied spirits, endowed with visual perceptions _only_, and asks what _their_ conception of outness and solid extension must be. Here further refinements in the interpretation of visual perception, and its organic conditions, which have not escaped the attention of latter psychologists and biologists, are hinted at. ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ Whether the data of sight consist of non-resembling arbitrary Signs of the tactual distances, sizes, and situations of things, is a question which some might prefer to deal with experimentally—by trial of the experience of persons in circumstances fitted to supply an answer. Of this sort would be the experience of the born-blind, immediately after their sight has been restored; the conception of extension and its relations found in persons who continue from birth unable to see; the experience (if it could be got) of persons always destitute of all tactual and locomotive perceptions, but familiar with vision; and the facts of seeing observed in infants of the human species, and in the lower animals.