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Leonurtk,, Vol. I I , p. 23x. ( Pergamon Press Ltd. 1978. Printed in Great Britain T E R MINOLOGY The terms defined below were selected from artists’ Articles and Notes published in the previous issue of Leonardo. Each definition is followed by an indication of the volume and page number of Leonurdo on which the term first appears. The terms were not found in the art dictionaries among the dictionaries listed in Leonurdo 11,68 (1978) or they are used in a new sense. Some are terms that have been carried over into the field of art from the natural and social sciences, from mathematics and from technology. Numbers in brackets. e.g. [ I I , 171. refer to the dictionaries mentioned above. The definitions are not presumed to be unassailable. The editors would, therefore, welcome the comments of readers on the definitions presented. 878. Coalescent, coalescing solvent, coalescing agent (in plastic^ wchnology)-A solvent added to a polymer emulsion (intended for use in water-base paint, for example) to Insure the formation of a coherent polymer film. The addition of an adequate amount of coalescent reduces the MFT to a required level. (1I, 94) (cf. Term 882: /Iiininiunifilni,fbrniinX tenipi~roturr. M F n 879. Color-difference formula (in cdor sc.irnc~e)-An internationally accepted formula that provides a number that expresses the difference between two colors. The calculation generally requires the three CIE tristimulus values for each of the colors, a s obtained in colorimetry. (11, 90) 880. Imaginary metaphor (in visual urr)--A kind of visual metaphor that refers to a resemblance between, say, a pattern seen in a material under a microscope and a pattern of something seen on a much larger scale in nature when the two patterns have no factual connection. Thus. a microscopic view might provide an illusion ofdepth and consequently the image may resemble an actual 3-dimensional object with which it has noconnection. (11, 100)(cl. Term XX5: r i d ni~tup/ior) 881. Langley / i nph.v.sic..s)-~-Aunit of radiation, for example of solar radiation. It is equal to one gram-calorie per square centimeter of irradiated surface. (11, 90; [W. H. Allen, cd.. Dic.tioriarI~ of Tcd~nicul Ternis /br Ac~rospcicoU s c (Washington. D.C.: National Aeronautics and Space Administration, 19651 [171) 882. Minimum film forming temperature, MFT (in puinr tc~hiiologI~, -The minimum temperature at which a polymer emulsion (for example in an acrylic emulsion paint) can be dried to form ii coherent film. At temperatures above the MFT the polymer particles dispersed in water coiilesce to form ;I continuous film when the water evaporates to dryness: at temperatures below the MFT the particles do not coalesce. (11, 94: [R. L. Feller. N. Stowlow and E. H. Jones. On Pic,/rrrc, Vurnishcv mid Tlioir Sol~cvii.s. 2nd ed. (Cleveland: Press of Case Western Reserve Univ.. 1971) p. 2211) 883. Permanency rating, PR /inpuinr techno/og,r) -A nurnerical lightfastness rating devised by H. W. Levison for artists’ pigments and paints. It compares the color change in a sample resulting from exposure to light with the change in a reference specimen (having marginally acceptable lightfastness) after being submitted to the same exposure. PR is calculated from measurements of color difference. A PR equal to 10 denotes IOOo/; lightfastness; equal to zero, marginal lightfastness: less than zero (negative), unacceptable lightfastness for products used in the fine arts. (11, 91: [H. W. Levison. Ar/i.srs’ Pignients: Ligh//us/ncw Tesrs mid Rarings (Hallandale. Florida: Colorlab. I Y76))) (cf. Term 879: c,o/f,r.-t/rffrrc,rrc,c. /orniu/u) 884. Photo-lithoprint, photo lithograph ( i n grupliic urr)-A lithograph prepared by a process employing photographically prepared plates. (1I, 122: [171) 885. Real metaphor (iri vi.sirulurr) -A kind of visual metaphor that refers to a resemblance between. say, a pattern seen in a material under a microscope and a pattern of something more familiar seen on a much larger scale in nature (for example an aerial view of a river system)when the two patterns have a factual connection (forexample they were formed by the same or related physical or chemical forces). (11...


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