Relative luminosity in the extreme red

Abstract

When an electromagnetic wave of a suitable frequency enters the eye it is followed in a short space of time by a visual sensation in the brain. This "visual process" can be divided up into stages, such as the conversion of the electromagnetic wave into a stimulus, the production and transmission of nerve impulses actuated by this stimulus, and the conscious sensation arising thereform. Although no direct evidence is forthcoming, it is generally believed that the first stage referred to is a photochemical one similar in its behaviour to the many photochemical reactions known to be produced by light of visible frequencies. This paper describes experiments in the extreme red and discusses their application to a photochemical theory of the primary visual process. It is generally assumed that vision extends on the red side of the spectrum to a wave-lengths of about 800 mμ , but some authors claim to have seen longer wave-lengths than this (840 mμ ). Nutting states that light of 1000 mμ is visible if sufficiently intense. In most of these observations account has not been taken of the width of the spectral range of the light used, nor has care been taken to obtain very pure light. It will be shown in this paper that, when working above 820 mμ , it is necessary to use a double monochromator, together with an "infra-red" filter to ensure that the light seen is not scattered light.