"These ocular spectra are of four kinds: 1st, Such as are owing to a less sensibility of a defined part of the retina; or _spectra from defect of sensibility. _ 2d, Such as are owing to a greater sensibility of a defined part of the retina; or _spectra from excess of sensibility_. 3d, Such as resemble their object in its colour as well as form; which may be termed"