232 PHILOSOPHICAL TRANSACTIONS. [aNNO ] 7 1 6. 



very intense, the experiment succeeded better with both prisms. All that were 

 present trying the experiment found it to succeed, and that every circumstance 

 answered to the account given in prop. 1, theor. l, book 1, of Sir Isaac New- 

 ton's Optics, as far as the directions there given were followed. So that it 

 appeared that the blue being carried lower than the red in the first case, and 

 lifted higher in the second, was owing to the greater refraction of the blue ray; 

 for though each part of the ribbon or worsted reflected all manner of rays, yet 

 the phenomenon was very apparent; as also that the blue ribbon or worsted 

 reflected the blue rays more copioussly than the red rays, and that the red ribbon 

 or worsted reflected the red rays more than the blue ones, because the red of 

 the blue half, seen through the prism, was less intense than that of the red 

 half, and the blue or purple of the red half, seen through the prism, was less 

 intense than that of the blue half. 



N. B. If the ribbon or worsted be laid on any enlightened body, the pheno- 

 menon will not succeed so well, the colours of the body, seen through the 

 prism, mixing with those of the ribbon or worsted. Even a black body will 

 not do, if light falls upon it; but there must be a black cloth behind, in such 

 manner, that no light falling on it can be reflected so as to disturb the pheno- 

 menon. And if a short-sighted person looks through the prism, a concave 

 lens between his eye and the prism will render the phenomenon more distinct 

 than it would otherwise be. 



Exper. II. — Some days after, the sun shining, I made two holes h, h, in the 

 window shutter s s, of a darkened room, fig. 3; through which letting the 

 sun*s beams pass, by means of two prisms a, b, one near each hole, I opened 

 the rays coming from the sun into the two coloured spectra a, (3, where the fol- 

 lowing colours were very distinct, viz. red, orange, yellow, green, blue, purple, 

 and violet. Now the reason of their being more distinct than ordinary, was, 

 that the prisms which I used were made of the greenish glass mentioned before; 

 which is very free from those veins by which the colours are too much thrown 

 into one another, by the best white prisms of the common sort. 



The forementioned coloured spectra being thrown into the room, to the dis- 

 tance of about 20 feet from the window where the sun's light came in, I caused 

 a piece of white paper tt, \ inch broad, and 5 inches long, to be held within 

 the refracted rays, at the distance of 10 feet from the windows, which produced 

 these colours in such manner, that by turning the prisms round their axes, I 

 could make the red ray of the spectrum, made by the one prism, fall on one 

 half of the paper, and the purple ray of the spectrum, made by the other 

 prism, fall on the other half; for the spectra were both vertical, the lines which 

 terminated their long sides towards each other, just touching, as appears in 



