VOL. VII.] PHILOSOPHICAL TRANSACTIONS. 2Q 



siderations. There are yet some particulars to be taken notice of before I con- 

 clude, as the denial of the Experimentum Crucis. On this I chose to lay the 

 whole stress of my discourse ; which therefore was the principal thing to have 

 been objected against. But I cannot be convinced of its insufficiency by a bare 

 denial, without assigning a reason for it. I am apt to believe it has been mis- 

 understood ; for otherwise it would have prevented the discourses about rarefy- 

 ing and splitting of rays: because the design of it is to show, that rays of divers 

 colours considered apart, do at equal incidences suffer unequal refractions, with- 

 out being split, rarefied, or any ways dilated. 



In the considerations of my first and second propositions, the animadversor has 

 rendered my doctrine of unequal refrangibility very imperfect and maimed, by 

 explicating it wholly by the splitting of rays ; whereas I chiefly intended it in 

 those refractions that are performed without that supposed irregularity; such as 

 the Experimentum Crucis might have informed him of. And in general I find, 

 that whilst he has endeavoured to explicate my propositions hypothetically, the 

 more material suggestions by which I designed to recommend them, have esca- 

 ped his consideration; such as are, the unchangeableness, of the degree of re- 

 frangibility peculiar to any sort of rays; the strict analogy between the degrees of 

 refrangibility and colours; the distinction between compounded and uncom- 

 pounded colours; the unchangeableness of uncompounded colours; and the 

 assertion, that if any one of the prismatic colours be wholly intercepted, that 

 colour cannot be new produced out of the remaining light by any further refrac- 

 tion or reflection whatsoever. And of what strength and efficacy these particu- 

 lars are for enforcing the theory, I desire therefore may be now considered. 



^?i Account of tivo Books. N° 88, p. 5103. 



I. Ottonis de Guericke * Experimenta Nova Magdebrurgica, de Vacuo Spatio, 

 &c. Ams. An. 1672, in fol. 



* Otto or Otho Guericke, counsellor to the Elector of Brandenbourg, and burgomaster of Magde- 

 bourg, was born in l602, and died at Hanibourg in l6"86. He was one of the best philosophers of 

 his time, and produced several useful inventions. Of these, one was the air-pump j and the two 

 brass hemispheres, which being applied to each other, and the air drawn out, \6 horses were not able 

 to draw them asunder j also the virunculus or marmouset, which descended in a tube against rain, 

 and rose again on the return of serene weather j besides several others: though the last machine fell 

 into disuse after the invention of the barometer. He was also the author of several other works on 

 Natural Philosophy, besides that noticed in the article above, which contains his experiments on tlie 

 vacuum. As Guericke made use of his marmouset to foretel storms, he was considered as a sor- 

 cerer by the people ; and hence, when tlie thunder had one day fallen on his house, and broken to 

 pieces some machines which he had employed in his experiments, they failed not to say it was a pu- 

 nishment from heaven, that was angry at his infernal dealings. 



