302 NATURE'S TEACHINGS. 



of water existed. The Rainbow is nothing but a vast spec- 

 trum, a transverse slice of which would be a good representa- 

 tion of the coloured band which is shown in the instrument. 

 It is prefigured in the ever-shifting rainbows of the water-fall 

 and fountain, which latter may even be seen in the fountains of 

 Trafalgar Square, while at the Crystal Palace their beauty has 

 long been noticed. 



There is not a dewdrop which is not a miniature Spectro- 

 scope, as it glitters with its wondrous iridescence in the rays 

 of the rising sun ; there is not an opal with its shifting hues, 

 nor the splendour of the soap-bubble, nor the nacre of the 

 common river mussel or the ormer shell, which does not owe its 

 beauty to the same principles which govern the Spectroscope. 

 Every green leaf, and blue or pink or yellow petal, every vary- 

 ing tint of the mackerel sky, every blaze of sunset and blue- 

 grey of sunrise, owes its beauty to those wondrous laws of 

 light which had been hidden for so many centuries, until they 

 were unveiled by the simple prism of the Spectroscope. As in 

 so many instances, the revelation lay concealed until the coming 

 of the revealer, whose inspired hand raised the dark veil ot 

 centuries. 



THE THAUMATROPE. 



MIDDLE-AGED persons will recollect that since the days of 

 their childhood a great variety of optical apparatus has been 

 invented ending in the word " trope." This is a Greek word, 

 signifying to turn, and is given to the instruments because 

 they revolve. 



All these toys and they may some day become more than 

 toys depend on a curious property of the human eye. The 

 reader will remember that in the description of the human eye, 

 as compared with the camera obscura as applied to pho- 

 tography, it was mentioned that the image was thrown from 

 the front to the back, and in the one case was received on a 

 naturally sensitive membrane, and in the other on a film ren- 

 dered artificially sensitive by chemical means. This mem- 

 brane is called the " retina," because it not only receives the 

 impression, but retains it for some little time after the object 

 is removed. It has been calculated that the duration of the 

 image is about the eighth part of a second. 



