'294 I'HILOSOPHICAL TRANSACTIONS. [aNNO 1728. 



with indigo; and then there will he a space between the two distinct bases, 

 where both the images will be indistinct. 



Fig. 2 represents the box with one side out, whose place is (JdbB, eg is the 

 hole for the tube in the door of the foreside, xJcd; rr the hole in the back to 

 receive the piece r with its painted paper. 



Fig. 3 is the box open before, with the candles and paper in it, the same 

 parts being marked with the same letters as in the other figures. 



The Doctor made the experiment in this manner, because Sig. Rizzetti attri- 

 buted the different foci of the colours to different inclinations, which could not 

 be alleged here; the red and blue being, as he had desired, successively fixed 

 in the very same place; so that, as the candles were fixed, the light fell on the 

 painted paper always with the same incidence. 



Exper. 2. — Instead of the red or blue paper at rr, fig. 1, 2, 3, the Doctor 

 fixed on the piece r, a paper half red, and half blue, as kb, fig. 4; then over 

 the hole in the fore part of the box, represented by eg, fig. 2, he fixed a square 

 plate >cJcd, fig. 4, with an oblong hole in it, 4 inches long in its horizontal 

 position, and J inch deep, through which might be seen the party-coloured 

 paper, as if it were only of the size and shape of this aperture, and strongly 

 enlightened by the candles hid in the box, the rest of the room being very 

 dark. 



The Doctor made this preparation, because Rizzetti objects to Sir Isaac's 

 first experiment of the first book, that the black cloth beyond the party-co- 

 loured paper, was not colourless, and therefore the experiment was not decisive 

 as particularly relating to the paper. 



KB, fig. 5, is the paper contracted in length and breadth, by the aperture of 

 the plate, which paper being viewed at the distance of 5 feet, by the prism 1, 

 appeared as drawn at rb. The prism being removed to 2, at the distance of 10 

 feet, showed the paper as at rb. And when it was at 3, at the distance of J 5 

 feet, the paper appeared as p|3. In these three cases, the blue, b, b, and /3, ap- 

 peared lower than the red, r, r, p, the refracting angle of the prism being down- 

 wards. When the refracting angle was held upwards, as at 5, then the blue a 

 was raised higher than the re^l r; but if due care be not taken, in turning the 

 prism, a reflection may be mistaken for a refraction, as at 4; and then indeed 

 the red and blue will be equally raised, as at t. 



This must have been Signior Rizzetti's mistake, when (in page 38) he says 

 that one colour was raised higher than the other by two lines, at 10 feet dis- 

 tance, but not at all at 5 feet; for several of the persons present at the Doctor's 

 experiments, made the same mistake at first, before they could perform the 

 experiment in the manner abovementioned ; which they at last did, and found 



