256 Professors Liveing and Dewar, On the circumstances [Oct. 30, 



stop should be placed here. The eye-glass should be adapted, in 

 form and position, to receive this image to the best advantage. 

 Not till then should the large focussing screw be used to bring the 

 stop, and practically the eye-piece, into its proper place. 



I believe I am justified in thinking that achromatism may be, 

 comparatively speaking, neglected : first, because it is allowed that 

 positive eye-pieces are not achromatic : secondly, because it seems 

 probable that chromatic aberration shews itself principally when 

 the lenses are large, which those of the eye-piece are not. I may 

 add that this kind of eye-piece seems to have been designed by its 

 inventor, Huyghens, to correct distortion, and not chromatic aber- 

 ration. 



(2) On the circumstances producing the reversal of spectral 

 lines of metals. By Professors Liveing and Dewar. 



Our object in investigating the reversals of the lines of terres- 

 trial elements has been to trace the parallel between the conditions 

 of the elements as they exist in the sun, and those in which we 

 can place them on earth ; with a view to illustrate more fully the 

 problems of solar chemistry. A knowledge of the reversible lines 

 may also help us to distinguish those rays which are directly due 

 to the vibrations communicated to the luminiferous ether by the 

 freely moving molecules themselves from those which are produced 

 by superposition of waves, or by some peculiarity in the mode in 

 which the molecules are put in motion, or by some strain upon 

 them, such as the violent Leyden jar discharge might give, which 

 would not be an easily reversible action. 



In a former communication we brought before the Society the 

 results of our observation of the reversal of the lines of some of the 

 more volatile metals when the continuous spectrum of the hot 

 walls or end of an iron or porcelain tube was observed through 

 vapour filling the tube. As the temperature of the tubes was only 

 that of a crucible furnace (about 1500° C. at the outside), it was 

 a very limited number of the lines even of the volatile metals 

 which could be reversed in that way, because the relative strength 

 of the emissive (and therefore of the absorptive) power of a vapour 

 for particular rays often varies considerably when the temperature 

 is changed. Thus the vapour of sodium in an ordinary flame 

 emits little radiation except that of the yellow pair of lines and 

 only absorbs the same ; and it is only at higher temperatures 

 such as that of the electric arc, or that of a flame locally in- 

 creased by the introduction of an endothermic salt such as 

 a chlorate, that the other pairs (or groups) of lines red, orange, 

 green and blue are seen either bright or reversed. We want 

 then a source of light which shall give a more or less con- 

 tinuous spectrum of an intensity greater than that of the lines to 



