342 PHILOSOPHICAL TRANSACTIONS. [aNNO iQyGi 



principal of the experiments I had tried ; amongst which there happened to be 

 the principal of those experiments which Mr. Lucas has now sent me. And as 

 for the experiments set down in my first letter to you, they were only such as I 

 thought convenient to select out of that tractate. — But suppose those had been 

 my whole store, yet Mr. Lucas should not have grounded his discourse upon a 

 supposition of my want of experiments, till he had examined those few. For 

 if any of those be demonstrative, they will need no assistants, nor leave room 

 for further disputing about what they demonstrate. 



, The main thing he goes about to examine is, the different refrangibility of 

 light. And this I demonstrated by the experimentum crucis. Now if this de- 

 monstration be good, there needs no further examination of the thing ; if not 

 good, the fault of it is to be shown : for the only way to examine a demon- 

 strated proposition is, to examine the demonstration. Let that experiment 

 therefore be examined in the first place, and that which it proves be acknow- 

 ledged, and then if Mr. Lucas wants my assistance to unfold the difficulties 

 which he fancies to be in the experiments he has propounded, he shall freely 

 have it ; for then I suppose a few words may make them plain to him : whereas 

 should I be drawn from demonstrative experiment to begin with those, it might 

 _ create us both the trouble of a long dispute, and by the multitude of words 

 cloud, rather than clear up the truth. For if it has already cost us so much 

 trouble to agree upon the matter of fact in the first and plainest experiment, and 

 yet we are not fully agreed; what an endless trouble might it create us, if we 

 should give ourselves up to dispute upon every argument that occurs, and what 

 would become of truth in such a tedious dispute ? The way therefore that I 

 propound, being the shortest and clearest, not to say the only proper way, I 

 question not but Mr. Lucas will be glad that I have recommended it, seeing he 

 professes that it is the knowledge of truth that he seeks after. And therefore 

 at present I shall say nothing in answer to his experimental discourse, but this 

 in general ; that it has proceeded partly from some misunderstanding of what he 

 writes against, and partly from want of due caution in trying experiments ; and 

 that amongst his experiments there is one, which, when duly tried, is next to 

 the experimentum crucis, the most conspicuous experiment I know, for proving 

 the different refrangibility of light, which he brings it to prove against. 



By the postscript of Mr. Lucas's letter, one not acquainted with what has 

 passed, might think, that he quotes the observation of the Royal Society against 

 me ; whereas the relation of their observation, which you sent to Liege, con- 

 tained nothing at all about the just proportion of the length of the image to its 

 breadth according to the angle of the prism, nor any thing more (so far as I can 

 perceive by your last) than what was pertinent to the things then in dispute, viz. 



