rjg Oil the Iiijltukn, R^JL-s'ioiiy and Colours of Light. 



niflics ; and \i D C be ftoppcJ, g the green vanifhcs'j and if D V be flopped, vdifappears. 

 Laflly, if D R and D G be ftopped, g and r vanifli. 



ObJ. VI. Having produced a fet of bright images, I let one pafs through tile delk dcfcriSed 

 in the third Experiment, and received it on a finall lens t inch broad to collccl the rays into 

 a focus, which I received on the chart by moving it a little on its hinge ; and by all the ob- 

 fervations I could make, and all the teds I could think of, it was white inclining to yellow, 

 and of the fame nature and conllituiion with the fun's direCl light : but if any ray was 

 flopped 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 inverted order. At the focus I held a reflecflor, and there were formed images of all 

 the fcven colours, as in tiie fun"s dire£l light (Exp. i.) ; if the light was fufficiently ftrong, 

 and the deilc near the window-fhut hole, one of thefe could even be collcifled by a fecond 

 lens into a wliite focus. This experiment is rendered more uniform by fubflituting for tlic 

 lens a concave metallic mirror, and placing at the focus another mirror to reduce the rays 

 into a beam which may be madeof any compofuion we pleafe by flopping one or more of 

 the colours at the hole in the defk. I obferved in the courfe of thefe experiments a phe- 

 nomenon worth mentioning; if a comb (as in Newton's experiment*) be very fwiftly 

 moved before one of the images or more, a fenfation of white is produced: but this is flili 

 more evident if the pin be fwiftly moved round its axis ; for then the images move alfo, 

 and, running into one another, caufe a fenfation of perfedl whitenefs. 



Obf. VII. I let an image through the hole in the dcfk, and viewed it through a glafs prifm, 

 holding its axis parallel to the fides of the image, and its refraining angle upwards. I 

 found that if the image was bright, and free from white light, the colours were not 

 changed by the rcfra£lion ; but if it was mixed and diluted with white, the prifm, decom- 

 pounding the white, caufcd the image to appear violet at one fide, and red > at the other ; 

 yet flill this only confufcd the colours of the image without changing them. Farther, if 

 the prifm was moved on its axis, the violet was lifted higher than the red or any of the 

 other colours. Nor was tHe conftitution of the colours at all changed by reflexion from a ' 

 pin or mirror, except in fo far as they were mixed by a concave one, as mentioned in the 

 laft experiment. If a pin was held behind the hole to refle£l the colours, it formed other 

 images of the colour in which it was held, and as far as I could judge threw the red to 

 the greateft diftance and breadth and inclination. Nor were the colours of the imagi 

 changed by reflexion from natural bodies, for thefe were all of the colours in whicli they 

 were held, but brighteft in that which they were difpofed to reflcci moft copioufly. Like- 

 wife the ring? of colour made by thin plates were broadeft in the red, and narrowed in the 

 violet ; and the like happened to the fringes that furround the fhadows of bodies. Laftly, 

 the (hadows of bodies were themfelves broadeft in the violet, and narrowed in the red. 



O.y. VIII, I filled with water a glafs tube, whofe diameter was ^th of an inch, and confe- 

 quently the radius of curvature ~xh, and whofe fides were ^'^th of an inch thick; then 

 danding at foiu feet from a candle I held the tube {th of an inch from my eye, fo that the 

 light, of the candle might be refracted through it, and moved my eye-lids clofe cqough to 

 prevent tlw; extvancous fcattered light from entering along with that whidi was regularly 



■ Optics, Book i-.Tart IX. Prop. 5. 



rcfraCled. 



