VII. The sensitiveness of the retina to light and colour

Abstract

In “Colour Photometry,” Part III. (‘Phil. Trans.,' A, 1892), a description was given of the apparatus employed for estimating the intensity of light of any colours which just failed to cause any sensation on the retina. In the following research a modified form of apparatus has been employed, and experience has shown it to be equally as accurate as and more convenient in many ways than that formerly used. When rotating sectors are employed with less than 4° of aperture, small errors in the reading of the graduated arc cause appreciable errors in the result. Hence, the more nearly the zero reading is approached, the less trustworthy are the determinations of the diminution of the total intensity, and this uncertainty affects all observations in which the light is reduced below 1/40 of its initial amount. The sectors have also the disadvantage of not being noiseless, and of requiring an electro or other motor to work them.