330 Rev. B. Powell's Remarks on a Statement by Dr. Thomson . 



ject, it would certainly appear, from this whole passage, as if 

 my investigations had consisted merely in making objections 

 to De la Roche's conclusions, and suggesting another expla- 

 nation of his facts; and that this explanation had been sub- 

 sequently opposed by the experiments of Mr. Ritchie, and, in 

 the opinion of the author, invalidated by them. 



Such, it appears to me, is clearly what the language of the 

 passage would imply. And if so, I must be permitted to say, 

 the impression conveyed is very erroneous. 



In the first place, as to my own researches, I have only 

 to hope, that, in order to form a fair judgement of them, the 

 reader will take the trouble of referring to the Phil. Trans. 

 1825. Part i., and 1826. Part iii., where he will perceive that 

 my conclusions are advanced as the result of experiment, and, 

 as such, bear the character of demonstrated fact. If they are 

 to be refuted, it must be by pointing out some fallacy in the 

 experiments, or by exhibiting others more accurate and satis- 

 factory in opposition to them ; and this, as far as I am aware, 

 has not yet been done. If the facts be admitted, the explana- 

 tion of De la Roche's results, and indeed of numerous others, 

 follows as a necessary consequence. 



I was very much surprised, that, in the passage referred to, 

 the experiments and conclusions of Mr. Ritchie should be de- 

 scribed as contradictory to mine, and invalidating my explana- 

 tion of De la Roche's ; when in fact, they do not even refer to 

 the same subject as mine, except only on one subordinate point ; 

 and that, one which has no reference to De la Roche's re- 

 searches. 



In the main portion of my experiments, what I have stated 

 respecting glass screens is certainly proved only for screens 

 of ordinary and sensible thickness. If an exception were 

 found when an extremely thin screen is employed, it would 

 not interfere with the conclusion in other cases. 



Such an exception has been contended for by Mr. Ritchie, 

 on the ground of some very delicate experiments. I failed 

 in verifying his conclusion. He objected that my trial was 

 not sufficiently delicate; and adopted the ingenious variations 

 upon his method which Dr. Thomson has described, confirm- 

 ing his original conclusion. Here this subordinate question 

 rests. We have at present only to state the general law of 

 non-transmissibility, with this single exception in the case of 

 infinitely [indefinitely ?] thin screens. 



But this conclusion has manifestly no connection whatever 

 with De la Roche's results ; nor with my explanation, nor 

 any other which may be given of them ; this particular case 

 being one which did not enter at all into his inquiries. 



I must 



