WAVELENGTHS IN THE INVISIBLE PRISMATIC SPECTRUM. 151 



each other, as will be seen, as soou as they are called on for information, in the region outside of 

 it, where they would be chiefly useful. 



The present work bus been preceded by a new map of the invisible prismatic spectrum, where 

 the abscisste were proportional to the deviations in a certain prism. (See Comptes Eendus de 

 I'Academie des Sciences Sep. 11, 1882, and Am. Jour, of Science, Vol. XXVI, Plate III.) Ami 

 the immediate object of this research is to pass from the arbitrarily si)aced prismatic scale belong- 

 ing to the particular prism in question, to the present map on the noruud and absolutely general 

 one (Plate XII), which was indeed also presented in the sauu> journals, but in advance of the present 

 detailed description of the means used to obtain it. (The drawings referred to are here given in 

 detail in Plate XII.) 



I should perhaps make the cautionary remark, that the general conclusions here offered, as to 

 the relation of wave-lengths and indices of refraction, have been drawn from the observations on 

 a single prism and have not been experimentally verified on others. This is on account of the ex- 

 tremely slow and laborious character of the process used (which has involved some months of labor 

 for this special prism). Though there seems no reason to doubt the generality of our conclusions, 

 it may be hoped that these experiments wiU be repeated with prisms of other material, and by 

 other observers, now that the preliminary obstacles have been removed. 



In order to map the spectrum on the normal scale, where the wave-lengths are equally spaced, 

 from such a map as that shown in Plate XII, in which the consideration of wavelengths does not 

 enter, it is necessary to establish some relation between the wave-lengths of rays and their devia- 

 tions, or between their wave lengths and refractive indices, which are connected with the devia- 

 tions by the well-known forrnula 



. '{a+d) 

 sin- '2 — 



n= — 5 



. a 

 sin^ 



where a=:the refracting angle of the prism, <Z = the deviation, and » = the corresponding index of 

 refraction. In the visible spectrum, the deviation, in any prism, of the Fraunhofer lines (whose 

 wave-lengths have been very accurately determined) can be measured by means of an eye-piece 

 with cross-wires ; and, from a sufficient number of such measurements, by making ordiuates propor. 

 tional to indices of refraction (or deviations) and absciss* proportional to wave-lengths, a curve 

 may be found whose equation is n — {tp) A or <Z = (cp) A, representing the required relation to any 

 degree of exactness. 



In the invisible spectrum the difiQculties are immensely greater and demand special means, 

 not only on account of this invisibility, but owing to the absorption by the prism and to its com- 

 pressing the rays. 



The prism here used was made by Adam Hilger, of London, and its optical properties are in 

 every way satisfactory. It is of a white flint, which has proved singularly transparent to the 

 longest so!ar waves. Its principal constants are : 



(1) Size of polished faces, 53»"" by 49°"". 



(2) Specific gravity, 3.25. 



(3) Refracting angle, 62° 34' 43". 



(4) Index of refraction (given in Table III). 



APPAKATUS 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 Rutherfukd reflecting grating (either of 681 lines to the 



