328 PHILOSOPHICAL TRANSACTIONS. [aNXO I7OI. 



Exper. IX. — Having made a hole h in the paper which received the coloured 

 spectrum, I suffered the red light to pass ; which being refracted by a second 

 prism, fell on another paper at t, where it appeared still red, whether seen 

 with the naked eye or prisms of different refracting angles. To the eye which 

 saw it through the prism v, it appeared indeed lower, as at t, but red, round 

 and unchanged. I made the experiment on all the colours, which by this 

 means appeared to be simple and homogeneal. See fig. 22, where the same 

 letters denote the lens, prism and first paper. 



Through the same lens and prism the spectrum was made to fall on a book ; 

 then through the prism f it appeared unchanged ; and the letters in the book 

 which crossed the spectrum, were as distinct as when seen with the naked eye. 

 See fig. 23. 



N. B. The axis of the prism p ought to be perpendicular to the long axis of 

 the spectrum sm thrown on the book, which will appear at o-/* ; and the prism 

 in the position represented at f, with its fiat side towards the nose : for that is 

 the most convenient position for looking at the spectrum in these experiments. 



I suffered the purple ray only to pass through the hole h and fall on a book at 

 p, the letters of which appeared at tt, and were as distinct through the prism q 

 as when seen with the naked eye ; and I had the same success with all the other 

 rays. See fig. 24. 



But if a sun-beam, as r, comes through the hole h directly on the book at w, 

 an eye looking at it through a prism at x will see this beam at y oblong and 

 coloured, and the letters on which it falls confused. See fig. 24. 



N. B. The lens ought to be very good, without veins or blebs, and ground 

 to no less a radius than mentioned in the experiment ; though a radius of a foot 

 or two longer is not amiss. The prism ought to be of the same glass as the 

 object-glasses of telescopes, the white glass, of which prisms are usually made, 

 being commonly full of veins. And the room in these last experiments ought 

 to be very dark. 



A few days after, having got very good prisms, made for the purpose, of the 

 above-mentioned glass, I made all the experiments over again before several 

 members of the Royal Society, with better success ; and had the spectrum very 

 regularly terminated, without any pale light darting from the ends of it. 



For a further account of experiments to this purpose, see Sir Isaac Newton's 

 Optics, b. 1 , part 1 , to which I might have referred the reader altogether ; but 

 that I was willing to be particular in mentioning such things as ought to be 

 avoided in making the experiments above-mentioned; some gentlemen abroad 

 having complained that they had not found the experiments answer, for want of 



