Page 464 from The Theory of the Photographic Process, "The illuminance of the photograph generally must be greater than that on the original scene if exact luminance reproduction is to be obtained. The reason for this requirement is that the semispecular reflections from shiny objects in the scene, such as hair, eyes, certain areas of the skin, jewelry, certain types of cloth, glassware, ceramics, fur shiny leaves, rippling water, and many metallic objects, usually have a luminance greater than that of a diffuse white object in the scene. If the diffuse white object were to be recorded at the maximum reflectance of the photographic paper (approx 90%), the shiny areas of the scene could not be reproduced at the required higher luminances. For true reproduction, therefore, the diffuse white objects in the scene must be recorded at a reflectance less than 90% so that the shiny areas in other objects can be recorded at 90% and thus have a luminance higher than that of the photographic record of the diffuse white objects. If for example, the diffuse white objects in a living room scene lighted with 100 foot candles are reproduced so that they will have a reflectance of 45% in a photographic print and the brighter, semispecular objects are reproduced at a reflectance of 90%, the illuminance on the photograph must be 200, rather than the 100 foot candles."
Later in the chapter, page 466, "The gradient in the middletone region was always greater than 1.00 (usually 1.10 - 1.20) for the preferred prints of all the scenes studied. Whenever the middletone gradient was less than 1.10, the prints were unanimously rejected as being "too flat". Whenever the density level of the prints was great enough so that the curves closely approached the 45 degree reference line, the prints were unanimously rejected because they appeared too dark."