VOL. LXXXVI.l PHILOSOPHICAL TRANSACTIONS. 733 



which I held a chart, and there received the spectrum inverted, the red being 

 uppermost, and the violet undermost ; holding the reflector at 2 feet from the 

 focus, and 4 from the chart, the images were formed just as before, only in- 

 verted, inclining towards the violet, of greater breadth towards the red, and more 

 distended towards the same quarter when the reflector was moved. 



Obs. 5. Things remaining as in the last part of the last experiment, at the 

 focus of the lens I placed a 2d prism, which refracted the rays into a white beam, 

 (Optics, book. 2, part 2, prop. 2), and this I received on a screen with a hole in 

 the middle, through which a small part of it passed, and fallmg on the reflector 

 placed behind, was formed by it into images, after the manner of the first experi- 

 ment, each having in regular order the 7 prismatic colours. One of the brightest 

 and most distinct I let pass through a hole in a 2d screen, and it fell on the chart. 

 I then caused an assistant to intercept the red rays between the first prism and 

 the lens, and immediately the red part of the image vanished ; and when the 

 violet was intercepted, the violet of the image vanished ; and if the green was in- 

 tercepted, the green was wanting in the image. In sliort, whatever colours were 

 stopped, the same were missing in the image. In fig. 6, the rays passing through 

 the hole c of the window ab, are refracted by the prism pmn, and separated into 

 Dv, DG, and DR, violet, green, and red; which being collected into a focus f by 

 the lens l, are there again refracted by a prism p'm'n', and formed into a white 

 beam abjnn, part of which is intercepted by the screen ss', and part passes 

 through the hole h, as Ah to h on the chart xyzw, and part is reflected by the 

 body og into a set of images which are received on a screen xu, and one of them, 

 rgv, let pass to wxyz ; but when an obstacle e stops dr, r the red vanishes ; and 

 if DG be stopped, g the green vanishes ; and if dv be stopped, v disappears. 

 Lastly, if DR and dg be stopped, g and r vanish. 



Obs. 6. Having produced a set of bright images, I let one pass through the 

 desk described in the 3d experiment, and received it on a small lens x inch broad, 

 to collect them into a focus, which I received on the chart, by moving it a little 

 on its hinge ; and by all the observations I could make, and all the tests I could 

 think of, it was white inclining to yellow, and of the same nature and constitu- 

 tion with the sun's direct light ; but if any ray was stopped before coming to the 

 lens, the focus was a mixture of the remaining rays ; and the chart being moved 

 a little farther round, the image was formed on it, the colours being in an in- 

 verted order. At the focus I held a reflector, and there were formed images of 

 all the 7 colours, as in the sun's direct light (Exp. l); if the light was sufficiently 

 strong, and the desk near the window-shut hole, one of these could even be col- 

 lected by a 2d lens into a white focus. This experiment is rendered more uni- 

 form by substituting for the lens a concave metallic mirror, and placing at the 

 focus another mirror to reduce the rays into a beam, which may be made of any 



