734 PHILOSOPHICAL TRANSACTIONS. [anN O 1/06 



composition we please, by stopping one or more of the colours at the hole in the 

 desk. I observed in the course of these experiments a phenomenon worth men- 

 tioning; if a comb (as in Newton's experiment, Optics, book J, part 2, prop. 5) 

 be very swiftly moved before one of the images, or more, a sensation of white 

 is produced ; lout this is still more evident, if the pin be swiftly moved round its 

 axis ; for then the images move also, and running into each other, cause a sen- 

 sation of perfect whiteness. 



Obs. 7- I let an image through the hole in the desk, and viewed it through a 

 glass prism, holding its axis parallel to the sides of the image, and its refracting 

 angle upwards ; I found that, if the image was bright and free from white lio-ht, 

 the colours were not changed by the refraction ; but if it was mixed and diluted 

 with white, the prism, decompounding the white, caused the image to appear 

 violet at one side, and red at the other ; yet still this only confused the colours 

 of the image, without changing them. Further, if the prism was moved on its 

 axis, the violet was lifted higher than the red or any of the other colours. Nor 

 was the constitution of the colours at all changed by reflection from a pin or 

 mirror, except in so far as they were mixed by a concave one, as mentioned in 

 the last experiment. If a pin was held behind the hole to reflect 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 greatest distance, and breadth, and inclination. Nor 

 were the colours of the image changed by reflection from natural bodies, for 

 these were all of the colours in which they were held, but brightest in that which 

 they were disposed to reflect most copiously. Likewise the rings of colours made 

 by thin plates were broadest in the red, and narrowest in the violet ; and the 

 like happened to the fringes that surround the shadows of bodies. Lastly, the 

 shadows of bodies were themselves broadest in the violet, and narrowest in 

 the red. 



Ob.s. 8. I filled with water a glass tube, whose diameter was J- of an inch, and 

 consequently, the radius of curvature 4, and whose sides were ~'^ of an inch 

 thick ; then standing at 4 feet from a candle, I held the tube 4- of an inch from 

 my eye, so that the light of the candle might be refracted through it, and 

 moved my eyelids close enough to prevent the extraneous scattered light from 

 entering along with that vvliich was regularly refracted. I saw several images of 

 the candle all highly coloured, and the colours were in order, from the candle 

 outwards, red, orange, and so on to violet ; I then filled the tube with clear di- 

 luted sulphuric acid, and dropped a small piece of chalk to the bottom, when 

 immediately an effervescence took place, by the escape of fixed air, which rose 

 in bubbles through the tube; and looking at the candle through one of these, I 

 saw the images formed with the colours still in the same order, but a little larger 

 than before. 



