USE OF LENSES 335 



During some volcanic eruptions a great amount of dust is thrown 

 into the air. The more dust in the air, the greater the loss of the 

 blue, and the greater the predominance of red and yellow. 



Sunset Colors. When the rays of light from the sun strike the 

 earth in a slanting position, the clouds act as prisms, separating 

 the light waves and allowing those colors to pass through which are 

 least turned from their course. Yellows, oranges and reds pass 

 through. The amount and variety of color depend, however, on 

 the thickness and height of the clouds. 



A Mirage. A mirage is an optical illusion which causes distant 

 objects below the horizon to be plainly seen. This happens in hot, 

 desert regions when the lower air near the ground js very much hotter 

 than the air above. The lower air, being expanded, is not as dense 

 as the cold air, which causes a ray of light traveling obliquely down- 

 ward to be refracted until the angle of refraction is more than 90. 

 Then total reflection takes place. Images seen are inverted. On 

 the Great Lakes, trees, boats and towns on the opposite shore sixty 

 or seventy miles away are sometimes plainly seen. The images are 

 usually erect, and are caused by the total reflection from the warm 

 still layers of air over the cold layers near the water. 



USE OF LENSES 



Lenses. Lenses may be considered a series of prisms, one 

 placed upon the other, and each one slightly different from the 

 others. 



Focus of a Lens. When the rays of light enter a lens they are 

 bent so as to meet at a point called the focus. The thicker the 

 lens the nearer the focus is to the lens, and the thinner the lens, the 

 further away the focus is from the lens. 



Effect of Object Distance on Images. The two principal features 

 of a convex lens are the principal focus and the axis. The image 

 produced by any lens depends upon where the object is placed in 

 respect to the principal focus. If the object is at a great distance 

 from the lens, the image is near the lens, and is small and inverted. 



This is the case with the eye, the view camera, and the object 

 glass of a telescope. If the object is twice the focal distance from 



