50 Prof. Tyndall on the History of Negative Fluorescence. 



of the rays, and the removal of the luminous portion of the radia- 

 tion. The second of these conditions constitutes the main diffi- 

 culty ; and we shall presently see whether the mode of surmount- 

 ing it proposed by Dr. Akin entitles him to compare his per- 

 formance with that of the mathematicians " who first conjectured 

 the existence of Neptune," and so on. He described before the 

 British Association, assembled at Newcastle in 1863, three experi- 

 ments. In the first of them he proposed to employ two conjugate 

 mirrors; at the focus of one of which he places a piece of 

 chalk or lime, and at the focus of the other an oxyhydrogen- 

 flame. He proposed to cut off " by absorbents " such visible 

 and ultra-violet rays as the flame emits. " Then," he says, " if the 

 mirrors are of sufficient sizie to render the temperature of the 

 distant focus approximately equal to that of the flame itself, there 

 is every reason to believe that the lime therein contained will 

 shine out." 



Six years of hard labour at these phenomena of radiation have 

 rendered such proposals rather amusing to me. The "tem- 

 perature at the distant focus " must, of course, be derived from 

 the rays emitted by the oxyhydrogen-flame, which are reflected 

 in a parallel beam by one mirror and concentrated by the other. 

 It never occurs to Dr. Akin to inquire what fraction of the heat 

 of an oxyhydrogen-flame is disposed of by radiation. He does 

 not at the present moment know whether the tenth, the hun- 

 dredth, the thousandth, or the ten thousandth part of the heat of 

 the flame is thus disposed of. What he imagines is plain enough — 

 namely, that, save some slight losses in his "absorbent" and at 

 the surfaces of his mirrors, the whole heat of the flame is radiated 

 against one mirror and condensed by the other. He entirely 

 forgets that a flame may be intensely hot, and its radiation 

 extremely feeble, and that this, in an eminent degree, is the case 

 with the oxyhydrogen-flame. It is not the practical difficulties, 

 which Dr. Akin himself discerns, that I am now speaking of; 

 it is the radical vice of the conception that a purely gaseous 

 flame, placed in the focus of a mirror, however large, could pos- 

 sibly generate a temperature u approximately equal to that of the 

 flame itself," in the focus of another mirror. 



In his second, and only rational experiment, Dr. Akin pro- 

 poses to concentrate the sun's rays by a concave mirror, and to 

 withdraw from the focus the luminous portion of the radiation. 

 But then comes the question, How is this to be effected? 

 Dr. Akin replies, " by proper absorbents." This, as far as I 

 know, constitutes his entire answer to the question. In all that 

 he has written upon the subject I have not been able to find a 

 hint of what the proper absorbent is to be. 



As a proposed experimental demonstration of a point which 



