338 PHILOSOPHICAL TRANSACTIONS. [aNNO I676. 



other red, l)oth severally transparent, yet both, if placed together, became 

 opaque. The reason whereof, says Mr. Newton, is, because if one liquor trans- 

 mitted only red, the other only blue, no rays could pass through both. In re- 

 ference then to this point, I filled two small glasses with flat polished bottoms, 

 the one with aquafortis, deeply tinged blue ; the other with oil of turpentine 

 coloured red; both to that degree, as to represent all objects through them 

 respectively blue or red. Then placing the one upon the other, I was able to 

 discern several bodies through both : whereas according to Mr. Newton's theory, 

 no object should appear through both liquors, because if one transmit only red, 

 the other only blue, no rays can pass through both. 



Mr. Neivtons Answer to the preceding Letter. N° 128, p. 698. 



Sir. — ^The things opposed by Mr. Line being upon trials found true and 

 granted me, I begin with the new question about the proportion of the length 

 of the image to its breadth. This I call a new one; for, though Mr. Line in 

 his last letter spake against so great a length as I assign, yet as it seems to me, 

 it was not to grant any transverse length shorter than that assigned by me, for 

 in his first letter he absolutely denied that there would be any such length : but 

 to lay the greater emphasis upon his discourse whilst in defence of common 

 optics he was disputing in general against a transverse image; and therefore in 

 my answer I did not prescribe the just quantity of the refracting angle with 

 which I would have the experiment repeated, which would have been a neces- 

 sary circumstance, had the dispute been about a just proportion of the length to 

 the breadth. Yet I added* this note, that the larger the angle of the prism is, 

 the greater will be the length in proportion to the breadth, not imagining but 

 that when he had found in any prism the length of the image transverse to the 

 axis, he would easily thence conclude, that a prism with a greater angle would 

 make the image longer, and consequently that by using an angle great enough 

 he might bring it to equal or exceed the length assigned by me, as indeed he 

 might; for, by taking an angle of 70or 75 degrees, or a little greater, he might 

 have made the length not only five but six or eight times the breadth, and 

 more. No wonder, therefore, that Mr. Lucas found the image shorter than I 

 did, seeing he tried the experiment with a less angle. 



The angle indeed which I used was but about 63° 12', and his is set down 

 60°, the difference of which from mine being but 3° 12', is too little to recon- 

 cile us, but yet it will bring us considerably nearer together. And if his angle 

 was not exactly measured, but the round number of 60° set down by guess, or 



* In my first letter in Philosophical Transactions, No. 121. 



