VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 379 



broad shadows ; and looking through this tube with one eye, while the other 

 was closed, I kept my attention fixed on the shadow, while an assistant repeatedly 

 interposed a sheet of yellow glass before the lamp whose light corresponded to the 

 shadow I observed, and as often removed it. The result of the experiment was 

 very striking, and fully confirmed my suspicions with respect to the fallacy of 

 many of the appearances in the foregoing experiments. So far from being able 

 to observe any change in the shadow on which my eye was fixed, I was not able 

 even to tell when the yellow glass was before the lamp, and when it was not ; and 

 though the assistant often exclaimed at the striking brilliancy and beauty of the 

 blue colour of the very shadow I was observing, I could not discover in it the 

 least appearance of any colour at all. But as soon as I removed my eye from the 

 tube, and contemplated the shadow with all its neighbouring accompaniments, 

 the other shadows rendered really yellow by the effect of the yellow glass, and 

 the white paper which had likewise from the same cause acquired a yellowish hue, 

 the shadow in question appeared to me, as it did to my assistant, of a beautiful 

 blue colour. I afterwards repeated the same experiment with the apparently blue 

 shadow produced in the experiment with day light and candle light, and with 

 exactly the same result. 



How fiir these experiments may enable us to account for the apparent blue 

 colour of the sky, and the great variety of colours which frequently embellish the 

 clouds, as also what other useful observations may be drawn from them, I leave 

 to philosophers, opticians, and painters to determine. In the mean time, I be- 

 lieve it is a new discovery, at least it is undoubtedly a very extraordinary fact, that 

 the eyes are not always to be believed, even with respect to the presence or ab- 

 sence of colours. 



I cannot finish this letter without mentioning one circumstance, which struck 

 me very forcibly in all these experiments on coloured shadows, and that is, the 

 most perfect harmony which always appeared to subsist between the colours, what- 

 ever they were, of the 2 shadows ; and this harmony seemed to be full as perfect 

 and pleasing when the shadows were of different tints of brown, as when one of 

 them was blue and the other yellow. In short, the harmony of these colours 

 was in all cases not only very striking, but the appearances were altogether quite 

 enchanting ; and I never found any person to whom I showed these experiments 

 whose eyes were not fascinated with their bewitching beauties. It is however 

 more than probable, that a great part of the pleasure which these experiments 

 afforded to the spectators arose from the continual changes of colour, tint, and 

 shade, with which the eye was amused, and the attention kept awake. We are 

 used to seeing colours fixed and unalterable, hard as the solid bodies from which 

 they come, and just as motionless, consequently dead, uninteresting, and tire- 

 some to the eye ; but in these experiments all is motion, life, and beauty. 



3 c'2 



