unrecognized Wave-lengths. 169 



confirm and greatly extend the results of 1882 and 1884. 

 Their most salient feature is still perhaps that already noted, 

 i. e. while the curvature, as far as we can follow it, grows less 

 and less, at the last point at which we can view it the curve 

 is not only all but sensibly a straight line, but one making a 

 very definite angle with the axis of X. This obviously means 

 that beyond this point n is nearly a linear function of A, or 

 that the simple equation n = a\ would very closely represent 

 this portion of the curve. It means also that, as far as these 

 observations extend, we find scarcely any limit to the index 

 of the ray which the prism can transmit except from its own 

 absorption. I do not, it will be observed, undertake to 

 advance without limit beyond observation, or to discuss what 

 would happen with wave-lengths so great that the index be- 

 came or negative, as it would with an indefinite prolonga- 

 tion of the curve, if its direction remain unaltered. An 

 intelligible physical meaning might perhaps be attached to 

 these cases ; but I here confine myself to the results of 

 direct observation and to the now established fact that the 

 increase of the crowding together of the rays at the red end, 

 which is so conspicuous a feature in the upper prismatic 

 spectrum, has almost wholly ceased, and that the dispersion 

 has become approximately uniform, the action of the prism 

 here being assimilated to that of the diffraction- grating itself. 

 I shall not venture to treat of the theoretical import of this, 

 further than to remark that the ordinary interpretation of 

 Cauchy's theory will apparently lead us to conclude that dis- 

 persion must sensibly cease at the point where the wave is so 

 long that the size of the components of matter is negligible 

 in comparison. In other theories, also, there appears to be a 

 point below which the index of refraction should never fall ; 

 and w T e might anticipate that the curve would accordingly 

 tend to become parallel to the axis of X. Of course we 

 cannot assert from observation that it will not finally do so ; 

 but within the very extended limits in which we have fol- 

 lowed it the contrary happens, and the curve presents a 

 continuously increasing angle with that axis. 



These results, then, in some material points are in contra- 

 diction to what has usually hitherto been believed *. 



Let me repeat that one consequence of the fact that the 



* I am very desirous that they should be verified by physicists, and I 

 have therefore given particulars in some detail of my methods and appa- 

 ratus here. 1 have requested the skilful artists (Mr. William Grunow, 

 Mechanician to the U. S. Military Academy, West Point, N.Y., and Mr. 

 J. A. Brashear, Allegheny, Penn.) who have so successfully constructed 

 this apparatus to place at the command of physicists all or any details 

 of it. 



