The Fundamental Principles of Vision in Very Weak Light

Abstract

A scale of subjective brightness is proposed in terms of which the changes in sensitivity of the eye can be recorded. The unit of subjective brightness is called a “brill,” and it is suggested that the scale may also prove satisfactory as a means of specifying contrast. An experiment is described which enables the changes taking place during dark adaptation to be appreciated in a very striking manner and by means of which the time required to adapt down to low levels of adaptation other than complete dark adaptation can be recorded. The effect of area on visibility is briefly discussed and the action of a glare source in depressing the sensitivity of the eye and delaying its recovery is described. A short discussion of the extent to which contrast is affected by a reduction both of illumination and of area follows, after which a new test for defective night vision is described and some preliminary results reported. In the final section a brief description of a portable binocular matcher that can be used as a “brillmeter” to measure subjective brightness is given.