The shadow-edge is coloured, because the relative dispersion, _n{v}_ − _n{r}_ ——————————————— (_n{v}_ and _n{r}_ being the refractive indices for _n_ the extreme violet and red rays respectively), of the vaseline differs from that of the dense glass. The dispersion of the glass is very high, and exceeds that of any stone for which it can be used. Certain oils have, however, nearly the same relative dispersion, and the edges corresponding to them are consequently almost colourless. A careful eye will perceive that the coloured shadow-edge is in reality a spectrum, of which the violet end lies in the dark portion of the field and the red edge merges into the bright portion. The yellow colour of a sodium flame, which, as has already been stated, is selected as the standard for the measurement of refractive indices, lies between the green and the red, and the part of the spectrum to be noted is at the bottom of the green, and practically, therefore, at the bottom of the shadow, because the yellow and red are almost lost in the brightness of the lower portion of the field. If a sodium flame be used as the source of illumination, the shadow-edge becomes a sharply defined line. The scale is so graduated and arranged that the reading where this line crosses the scale gives the corresponding refractive index, the reading, since the line is curved, being taken in the middle of the field on the right-hand side of the scale. The refractometer therefore gives at once, without any intermediate calculation, a value of the refractive index to the second place of decimals, and a skilled observer may, by estimating the tenths of the intervals between successive divisions, arrive at the third place; to facilitate this estimation the semi-divisions beyond 1·650 have been inserted. The range extends nearly to 1·800; for any substance with a higher refractive index the field is dark as far as the limit at the bottom. [Illustration: FIG. 17.—Method of Using the Refractometer.] [Illustration: FIG. 18.—Scale of the Refractometer.] [Illustration: FIG. 19.—Shadow-edge given by a singly refractive Substance.]