THE SOAP-BUBBLE. 



99 



diminishes. The study of soap-bubbles and of the effects of 'the light is 

 very interesting. Newton made the soap-bubble the object of his studies 

 and meditations, and it will ever hold its place amongst the curious pheno- 

 mena of the Science of Optics. But before going into all the phases of 

 Lights and Optics we will proceed to explain the structure of the eye, as it 

 is through that organ that we are enabled to appreciate light and its marvel- 

 lous effects. 



It is often considered an embarrassing matter to fix precisely the 

 value of two lights. Nothing, however, can be easier in reality, as we will 



Fig. 92. Grotesque effects of curved surfaces. 



show. In comparing different lights, it is necessary to bear in mind the 

 amount of waste, the colour of the light, the luminous value of the source, 

 and the steadiness of the flame. The luminous value of a lamp-burner is 

 generally equalled by that of a wax candle, and we will take as an example 

 one of those at six to the pound. Very precise appliances are used for 

 this experiment when great exactness is required ; but it is easy to calculate 

 in a simple manner the differences in ordinary lights. Supposing we desire 

 to test the value of light given by a lamp and a wax candle, they must both 

 be placed on the table at an equal height, B andQ, (fig. 93), in front of 

 some opaque body, A, and then a large sheet of paper must be fixed as 

 vertically as possible to form a screen. When B and QL are lighted, two 

 shadows, E and F, are produced, to which it is easy to give exactly the same 



