172 = 8. P. Langley— Determination of Wave-lengths 
wave-lengths are equally spaced, from such a map as that — 
shown in Plate III (this Journal, vol. xxv), in which the con- 
sideration of wave-lengths does not enter, it is necessary to 
establish some relation between the wave-lengths of rays and 
their deviations, or between their wave-lengths and refractive 
indices, which are connected with the deviations by the well- 
nown formula, 
and its optical properties are in every way satisfactory. * 
of a white flint, which has proved singularly transparent to the 
Apparatus for Measuring Obscure Wave-lengths. 
In 1882, an apparatus was employed in which invisible rays 
after passing through the Hilger prism, at a known deviation, — 
fell on a Rutherfurd reflecting grating (either of 681 lines to 
_ Several determinations were thus ‘made of wave-lengths it ¢ 
the upper part of the infra-red, where the heat is relatively = 
confidence. 
