664 



THE POPULAR SCIENCE MONTHLY 



vessels appear repeated in the sky, or when distant land that is really below the 

 hoi-izon 'looms' \ip and glimmers upon the horizon in trembling headlands. 

 This illusion is called the mirage, and takes place when refraction exceeds the 

 critical angle and becomes reflection. 



" Fill a clear glass tumbler with water, and put a spoon in it, or dip one 

 finger in the water, and hold it above your head, so that you can look into the 

 water from below. You will find that you cannot see through the water up 

 into the air above. The under-surface of the water will appear to shine like 

 burnished silver, and the spoon or your finger will be reflected in it, as in a 

 beautiful mirror. This illustrates total reflection, and shows that in this case 

 all light thrown upward through the water is reflected from its surface. Look 

 into the tumbler from above, and it appears full of clear water. Look into it 

 from below, and it seems as if an opaque sheet of silver rested on the water, and 

 shut out the view of everything above." 



Fig. 7 shows a neat and simple arrangement by wbich water can 

 be nsed for a lens to illustrate refraction. It is merely a long pine 

 box fourteen inches higli, made of thin boards, with one side open, and 



a round hole in the top five inches 

 in diameter. On this opening rests 

 a hemispherical glass dish made by 

 cutting off the round top of a glass 

 shade, and which is filled with clear 

 water. A piece of looking-glass is 

 supported below at an angle of 

 forty-five degrees, so as to reflect a 

 stream of sunlight upward through 

 the water-lens. The rays are thus 

 refracted and brought to a point at 

 a proper distance above. If a sheet 

 of paper is held horizontally just 

 over the bowl, it will be lighted 

 up by the rays coming through the 

 lens. Raise the joaper slowly, and 

 the circle of light on its surface 

 will grow smaller and brighter, till 

 it is reduced to a point, when it 

 will burn a hole in the jDaper. If 

 a little smoke is diffused through 

 the space, it will reveal tlie double 

 cone of light, with one base upon 

 the surface of the water-lens, and the other forming a large circle of 

 light upon the ceiling, the rays all crossing at the focal point. With 

 some additional attachments Prof. Mayer makes such a lens work in a 

 magic lantern for projecting large pictures upon screens, the whole 

 mechanism being estimated to cost but |3.20. 



One of the most beautiful experiments in total reflection is that 

 illustrated by Fig. 8. A Florence flask filled witli water acts as a 



Fig. 7. The Water-Lens. 



