52 Prof. Tyndall on the History of Negative Fluorescence. 



As already stated, the obscure radiation from the electric light 

 has occupied my attention, more or less, during the last ten years. 

 In 1854 I sought by " proper absorbents " to separate the lumi- 

 nous from the non-luminous radiation of this source. In 1858 

 I again tried to do so ; and it was only a few days ago that the 

 last remnant of the black glass, prepared for me for this express 

 purpose by the late Mr. Darker, went to pieces in the condensed 

 beam of my electric lamp. It is not my habit, nor do I think it 

 a commendable habit, to sit down and propose experiments 

 which may, or may not, be capable of realization. At all events if 

 this be done at all, it ought to be done in a magnanimous spirit. 

 The true experimenter knows how frequently the most promising 

 ideas are shattered when he tries to realize them. He is forced 

 to be an iconoclast from day to day, breaking down the idols on 

 which his hopes were fixed only to be frustrated. Such a man is 

 not likely to sit down and write out experiments at his leisure, 

 with a view to mounting the high horse of "Neptune," and 

 claiming the credit, should anything similar be afterwards exe- 

 cuted. My own experience of an experimenter's difficulties — 

 difficulties which apparently had never dawned upon Dr. Akin 

 when he made his experiments on paper — are referred to in my 

 book on Heat, page 333. This same book, which is an account 

 of lectures given at the Royal Institution a year and a half 

 before the scientific advent of Dr. Akin, shows me in the act of 

 employing such "proper absorbents" as were then known. At 

 page 307 I describe experiments in which, by means of a plate 

 of rock-salt, coated with the smoke of a lamp, I cut off the lumi- 

 nous portion of the beam from the electric light. I mark, by a 

 rod, the focus of the invisible transmitted rays, and, bringing my 

 thermo-electric pile into this focus, cause the heavy needles of 

 my coarse galvanometer to dash violently against their stops. A 

 similar experiment, with black glass, is described at page 308. 

 But, though this substance transmitted obscure heat much more 

 copiously than the lampblack, the " absorbents " which first filled 

 me with hope, if not with confidence, are mentioned at page 351. 

 These absorbents are bromine, and a solution of iodine — sub- 

 stances suggested primarily by my own researches on the 

 deportment of elementary bodies towards radiant heat*. 



* Many years ago I was accustomed to explode a mixture of chlorine 

 and hydrogen by placing a lens in front of the electric light and a mirror 

 behind it, causing the foci of both to coincide within the flask which 

 contained the mixed gases. Twenty months before Dr. Akin appeared at 

 Newcastle I used substantially the same arrangement in attempting to 

 obtain an intense focus of invisible rays. It was my friend Dr. Debus 

 who, in answer to my inquiry about a proper solvent for iodine, proposed 

 bisulphide of carbon. I have since tried many other solvents, but have 

 found none so good. Could we obtain carbon in a state of solution, it also 

 might be found highly pervious to the ultra-red waves. 



