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Summary.-Animal research has adopted no standards for lighting practices. One consequence is that light intensity is either unreported or often improperly measured, even in experiments primarily concerned with light. This article tells how to measure and report light intensity and discusses the relative advantages of different kinds of light sources and lighting geometry. Roughly a hundred articles about the motivating properties of light have been published in the last decade. In a few of these, light intensity was reported in a helpful way (e.g., Stewart, 1960). But the vast majority of articles reveal a most casual concern for the stim~ilus whose effects were allegedly investigated. One often finds either a conspic~io~is absence of statements about lighting geometry and light intensity or an amateur discussion of irrelevancies. Also common is a meaningless confusion in which a photometric term is paired with an incompatible unit, as illustrated in a paper (reference withheld) stating "The intensity of illumination was one millilambert." The net result of chis variety of shoddy measurement and reporting practices is to make private what should have been public, namely, the physical value of the stimulus. Since it is quite clear that different light intensities produce different behavioral effects (Lockard, 1963), the "location" of the experiment along an easily measured continuum should be divulged. If it is not, the results of the experiment are not anchored to stimulus conditions and merely demonstrate that a particular effect can occur at some unknown and unduplicable combination of conditions. The experiment cannot be fitted into the fabric of scientific knowledge and cannot be replicated; and the results are, therefore, safe from expose if they are a Type I error. Any collection of such studies soon generates apparent but unresolvable inconsistencies, unresolvable because levels of variables other than light intensity are usually in doubt when one compares different studies. The artful reviewer can juggle the discrepancies in a multidimensional private calculus of guesswork, attaching effects to conditions; but this was the experimentalist's job. Had the experimentalist appreciated the rudiments of photometry, subsequent literary efforts could turn from guesswork about treatment effects to informative summarizing. 'The preparation of this paper was made possible by Grant No. HD 00942 from the National Institute of Child Health and Human Development. The content of the paper is largely from Chapter I, Basic Methodology, by J. B. Sidowski and R. B. Lockard, in Experimental Methods and lnstrumen!a!ion in Psychology, edited by J. B. Sidowski, McGrawHill, New York, 1966. The author is grateful to both J. B. Sidowski and the McGrawHill Book Company for their permission to draw upon copyrighted material.