Hess and Pretori revisited: Resolution of some old contradictions

Abstract

An early experiment by Hess and Pretori (1894) was replicated and modified in an attempt to determine why they failed to find the ratio principle later discovered by Wallach (1948). Separating the two surround-infield patterns by darkness made very little difference. However, allowing the observer to adjust the infield luminance (as in Wallach) rather than the surround luminance (as in Hess & Pretori) revealed some startling effects. At surround:infield luminance ratios greater than approximately 100:1, there is no ratio effect; all infields appear equal and totally dark. Converging evidence is presented that Hess and Pretori's data in this region actually represent surround-matching by the observers. Nor are ratio effects found with increments (infield brighter than surround). When free to match either infield luminances or ratios (by controlling infield luminance), observers match luminances. For decrements with ratios between 1:1 and approximately 100:1, lightness constancy and the ratio principle hold.One of the most extensive experiments in simultaneous contrast was published by Hering's students Hess and Pretori (1894/1970). In this early study, which calls to mind the more modem work by Wallach (1948), observers were presented with two adjacent infield-andsurround patterns, each pattern consisting of a 1.1 0 infield square surrounded by a 11.40 square background region. Each of the four regions was a white surface oriented at a 45 0 angle to the observer's line of sight so as to reflect light from a petroleum lamp that moved along a blackened tunnel at right angles to the observer's line of sight (see diagram in Evans, 1948, p. 165). This elegant method allowed good and independent control of, and a valid determination of, the four luminances at a time that predated photometers. The actual slanted position of each of the four regions was not perceived, since each was visible only through a mask containing an aperture that projected a rectangular region to the viewpiont of the observer, with the surround region serving as the mask for the infield.The infield-surround pattern on the left side was used primarily as a standard, and, for a given series of matches, the luminances of the left-hand infield and the left-hand surround were held constant. Within such a series, the right-hand infield was set at different luminances, and, for each, the observer was required to adjust the righthand surround luminance until the two infields appeared equal in brightness.Correspondence may be addressed to A. Gilchrist at the Department of Psychology, Rutgers University, Newark, NJ 07102. 7In all, the Hess and Pretori study used luminances ranging from 0 to 5,000 units (one unit was equal to 0.12 Hefner-Altenech units, or 0.01 tL), and tested luminance ratios from less than 1:5,000 to 256:1.Hess and Pretori's results did not show, as Wallach showed later under somewhat different conditions, that a given luminance ratio on the standard side was matched by the same ratio on the comparison side. What they did show, however, ...

Measuring surface achromatic color: Toward a common measure for increments and decrements

“…Although interesting,Gilchrist's approach relies on the untested assumption that matches on the incremental, emitting patch will form a perceptual continuum with the decremental patches. This assumption is challenged by a number of other papers, some by Gilchrist himself (Jacobsen & Gilchrist, 1988a), suggesting that lightness and brightness are separate continua. In addition, it is not clear how the hybrid scale could be applied on displays that appear under a homogeneous illumination so that perceived reflectance and perceived luminance are completely correlated.…”

“…In addition, it is not well understood how these matches relate to those on simple incremental or decremental scales. Finally, in several published works, Gilchrist (Jacobsen & Gilchrist, 1988a, 1988b) collected matches on a hybrid scale consisting of a set of decremental patches on a white surround plus an adjustable self-emitting region, also on white. The self-emitting region, constructed by adding a small chamber containing an adjustable light sending diffuse illumination through a translucent patch, effectively extended the lightness scale by providing a means for matching surfaces that were too bright for any of the decremental patches.…”

Brightness contrast, brightness constancy, and the ratio principle

“…Hess and Pretori visited once more. In their discussion of Hess and Pretori's (1894) data, Jacobsen and Gilchrist (1988b) begin by wondering why Hess and Pretori did not discover the ratio principle that they credit to Wallach (1948). There is really no puzzle, because the results…”

“…Under these circumstances, the test-field appears dark gray or black. Jacobsen and Gilchrist (1988b) argue that such data as those represented by lines 2,3, and 4 must represent an elaborate artifact, because the test fields under consideration all appear black, and further reduction of each test field's luminance will not make it appear any blacker. The situation may be clarified by reference to Figure 2 -co -2 -1 0 +1 +2 LOG INDUCING FIELD(mll Figure 2.…”

“…The value of the absolute threshold, measured inside the hole in the annulus, is known to increase systematically with the luminance of the annulus (Cobb, 1916). The problem with which Jacobsen and Gilchrist (1988b) concern themselves has its origin in the mistaken assumption that the values of these absolute thresholds fall along a constant brightness curve. That this is not so can be seen in Figure 3.…”