236 PHILOSOPHICAL TllANSACTIONS. [aNNO lfl6. 



I tried the same experiment with a lens of one foot radius, also with one of 

 9 inches, and with another of 7, and the success was the same. See fig. l6, 

 where r, o, y, g, b, p, v, express the colours. 



N. B. Care must be taken that the very end of the red, and the extremity of 

 the violet be taken in by the lens ; otherwise the spectrum will not be perfectly 

 white at the glass's focus. There is no fixed distance of the prism from the 

 lens ; but it ought to be brought so near the prism, that the two ends of the 

 •spectrum may fall nearer the axis of the lens, than the edges of the lens ; be- 

 cause there the refraction is not so regular. Behind the lens l, which made the 

 colours converge into white at the distinct base, or focus f, I placed the lens 1, 

 which made the white be at f the distinct base of the two glasses combined; and 

 the experiment succeeded as before, as in fig. 17. 



When the paper was held in the focus of the lens, so as to receive the white 

 image of the coloured spectrum projected by the lens ; if with a card I inter- 

 cepted the red ray, the white appeared tinged with purple ; and if I intercepted 

 the violet or purple ray, or both, the white appeared tinged with red; and if the 

 red was intercepted at the same time, the spectrum appeared to be a mixture of 

 yellow, green and blue. If any single colour was suffered to fall upon the lens, 

 the rest being intercepted, that colour would continue the same; only it would 

 be more intense in the focus of the lens. 



Exper. VII. — I took a board qhs, fig. 18, which stood reclining on a prop t, 

 having a hole of a quarter of an inch diameter at h, and behind it a prism b 

 supported on two props, as above-mentioned, so as to turn easily about its axis; 

 and having set this board on the ground with the prism behind it at b ; by 

 turning the prism AC about its axis, I first made the red ray of the coloured 

 spectrum pass through the hole h, and fall obliquely on the second prism b. 

 This ray, after its refraction in passing through the second prism, was carried 

 up to the ceiling of the room at the place marked r : I then made the purple 

 ray fall on the board, and pass through the hole h, as the red had done before; 

 and after refraction through the prism b it was carried up to the ceiling at p. 

 And the green ray being afterwards made to pass the second prism in the same 

 manner, went up to g : and so of all the intermediate rays, which were by this 

 second refraction thrown to the intermediate places on the ceiling between r 

 and p. 



Care is to be taken that the second prism be placed obliquely to the rays 

 which come through the hole h, lest they be reflected, as they would be, if 

 the board being in the position as, and the second prism in the position lnm, 

 the ray from the first prism be ^h ; for then it will be reflected upwards to (t, 

 instead of being refracted, as fig. 19. Neither must the plane of immersion be 



