VOL. XXXV.] PHILOSOPHICAL TRANSACTIONS. '2Q3 



Exper. 1. — The Doctor prepared a box, represented fig. I, pi. 6, like a 

 truncated pyramid, of about 3 feet high, and 1 foot wide within, in the 

 following manner: he painted the inside black, and in the back part, ] foot 

 above the base, made a square hole of 3 inches in width, whose section is rr, 

 to receive a piece r shutting close with a rabbet or shoulder, whose surface 

 coming through the hole was wholly covered with the painted paper, on which 

 the experiment was to be made. Opposite rr, in the fore part of the box, was 

 a door to open, with a tube in it, 4 inches wide, and 5 inches long, its section 

 e,f,g,h, that two candles set on the places i, k, to enlighten the paper at r,r, 

 might throw no direct light out of the box, whose section is represented at 

 abed. Then having made the room perfectly dark, he fixed the box on a table, 

 that it might remain in one place; at the distance of 8 feet from rr, he fixed 

 the lens ll, of 4 feet focus, in a frame on another table, with its axis going 

 through the middle of rr: at the distance of about 8 feet beyond the lens, he 

 set up the screen or square of white paper s. Having put into the iiole rr a 

 stiff paper, painted with vermilion, and wrapped four times and a half with 

 black silk, as represented by r, that paper enlightened by the candles at i, k, 

 the image of the red paper was projected on the screen at p, and when the rhost 

 distinct place was found, the screen was fixed: then a paper painted with ultra- 

 marine being fixed in the hole rr, its image was so indistinct at p, that the 

 images of the black silks could not be seen ; but holding a piece of paper close 

 to the screen, and bringing it forward, at about -I of an inch from it, the 

 representation of the silks began to appear on the blue image; but it was most 

 distinct at If inch, oratzz: so that there was 1^ inch between the distinct 

 base of the red, and that of the blue paper. 



But what has led several people into an error in making this nice experiment, 

 is the depth of the focus of the rays in both cases; for though the red image 

 was most distinct at p, yet the representation of the black silks might just be 

 perceived by a good eye when the screen was moved backwards or forwards -f- of 

 an inch; the blue image, which was stronger, had its silks visible an inch on 

 either side of zz; so that in a paper half red and half blue, painted with these 

 colours, one might have seen the silks, though faintly, on the two images at 

 once, and have been thereby deceived; but 4 of an inch beyond the place 

 common to both, the red alone would have appeared distinct; and an inch 

 short of the said place, the blue image most distinct, and distinct alone; that 

 is \\ inch nearer the glass. Instead of vermilion, the red paper may be painted 

 with carmine or lake, but it will not do so well, as was then tried, nor does 

 Prussian blue do so well as ultramarine. The best way is to heighten the ver- 

 milion with a little carmine, and the ultramarine, which has too much white. 



