Prof. Tyndall on the History of Negative Fluorescence. 51 



can only be decided by experiment, Dr. Akin's third proposition- 

 is, if possible, more hopelessly absurd than his first. 



These are the " ideas " of Dr. Akin, which I would have gladly 

 let him enjoy, had he permitted me to do so. The fact is, that 

 although he can fairly claim the credit of first proposing, in 

 public, a definite series of experiments, with a view to the solu- 

 tion of this question, his Note compels me to state that other 

 and more capable investigators, than he has proved himself to 

 be, abstained from proposing such experiments, simply because 

 they saw more clearly than he did the difficulties involved 

 in the practical treatment of the problem. This, I imagine, 

 sufficiently accounts for a fact which appears to have taken 

 him by surprise, namely, that notwithstanding his having pro- 

 posed so "simple" a plan, "he had vainly endeavoured, for 

 nearly a year, to procure for it a practical trial." The wished- 

 for opportunity at length came, and the practical trial was made. 

 Concentrating the solar rays by a concave mirror 18 inches in 

 diameter, Dr. Akin found that, when no absorbent was intro- 

 duced, a piece of platinum -foil, held in the focus of the mirror, 

 was rendered incandescent. But when he introduced, between 

 the mirror and the focus, a piece of monochromatic red glass, 

 " which of all substances, capable of absorbing the more refran- 

 gible part of the spectrum, allows the less refrangible part the 

 freest access, the incandescence was found to be extinguished, or 

 at least to become so faint as to be of doubtful visibility " (Reader, 

 September 26, 1863, p. 349). It is needless to remark that, 

 even had this experiment succeeded, the question would have 

 still remained unsolved ; for a sheet of glass, which permits the 

 most powerful rays of the visible spectrum to pass through it, 

 could not be called a " proper absorbent." He was afterwards 

 joined by the excellent Assistant Secretary of the British Associa- 

 tion, and in his article in the € Saturday Review ' expressed the 

 hope, "that in the course of next summer he will bring his 

 experiments to a successful termination." The summer came, 

 and a better one for his purpose rarely favoured England 

 — strong sunshine, and plenty of it ; and what is the result ? 

 Failure, but no abatement of pretension. " I have no doubt," 

 he says, " that, with the means at his command and his experi- 

 mental proficiency, Prof. Tyndall will now realize and c publish ; 

 a discovery which I have assigned the methods for accomplishing, 

 and which I should have probably effected myself, I may say, 

 years ago, had I been seconded as I had hoped, either by persons 

 or by circumstances." Dr. Akin knows perfectly well how safe 

 it is to boast " now " that he could have made the discovery 

 referred to. He has the best possible reason for having 

 "no doubt," viz. the sie^ht of his eyes. 



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