WHICH MICHEL-LÉVY CHART TO USE

Michel-Lévy charts are included in almost all textbooks on optical crystallography, although they are not always in color.  We will consider here only the colored ones.  Besides the color charts in optical crystallography textbooks, most manufacturers of polarized-light microscopes supply full-color Michel-Lévy charts, including Olympus, Nikon, Zeiss, Leitz, and, formerly, Vickers.  Typically, the polarized-light microscopist has the interference color chart framed, laminated, or otherwise mounted on a wall next to the microscope where it can be conveniently and quickly consulted.  The more one uses the interference color chart, the more useful and familiar it becomes; its potential application to the analysis of transparent substances is virtually unlimited.

Are there differences between the interference color charts produced over the last 115 years?  Which is the best?  Let’s conduct a more-or-less chronological survey of Michel-Lévy interference color charts, starting with the granddaddy of them all, the original one that appeared in 1888, which we already introduced as Figure 2.  This “Tableau des Biréfringences” has thickness to 60 µm, and includes four-orders of interference colors.  A birefringence value of 0.040 marks the upper right corner of this chart.  Mineral names are located as close as possible at their characteristic birefringence value.  There are very subtle color differences in the first-order gray-white area of the chart, and it is a great help to have the descriptive name of the color.  In the case of this original chart, there is a narrow strip of all four orders of interference colors along the top of the chart, together with the descriptive name of specific values of retardation.  Figure 5 is a close-up of the upper-left corner of this chart that illustrates this feature.  At about 24" X 18" in size, this is one impressive piece of work, and the opportunity to view one in person should not be missed.

click image to view large PDF version (972K) figure 5

It was mentioned that the Third Edition of Rosenbusch’s famous text, Mikroskopische Physiographie, was already published when Michel-Lévy’s book came out in 1885, but by 1904 it had been expanded to a two volume, Fourth Edition (16) in collaboration with E.A. Wülfing.  Volume I, called the first half, is devoted to morphology, and the principles of optical crystallography; the second volume contains the data on specific minerals.  In this edition, Plate 3 (Tafel III), the Michel-Lévy chart in color is mounted at the back of Volume I; Figure 6 is an illustration of this chart.  In the text (p. 283), it is referred to as “Die Michel-Lévysche Farbentafel” (The Michel-Lévy Color Table), but the actual color chart (Figure 6) is titled “Doppelbrechung und Jnterferenzfarbe” (Double Refraction and Interference Color).  This chart is 15" X 9¼"; the colored area is 10" X 4".  This chart indicates thickness to 50 µm; three orders of interference colors, and just the start of the fourth order; the vertical lines are at 50 nm apart, with every 100 nm numbered; birefringence 0.036 is at the upper-right corner; mineral names are located at their characteristic birefringence, and the interference colors are named.  This ~100 year old color chart is perfectly usable today.  It is printed on card stock, and folds at two places to fit between Tables 2 and 4.

click image to view large PDF version (797K) figure 6

Page 4 of 12

<<PREV | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | NEXT>>