MIRRORS. 71 



moment : the fire sparkling like that of a forge to which the 

 blast of a bellows is applied. 



The image of an object when received through a convex 

 lens will be inverted. If you cause the rays of light from 

 the flame of a candle to pass through the glass of a common 

 spectacle, and receive them on a sheet of paper, or dark 

 skreen placed at a proper distance, you will see a complete 

 inverted image of the candle on it. A convex lens placed 

 in the hole of a window-shutter will exhibit, on a white sheet 

 of paper situated in the focus of the glass, all the objects on 

 the outside, as fields, trees, men, and houses, in an inverted 

 order. The room should be quite dark, and the sun should 

 shine upon the objects. A portable camera obscura may be 

 made with a square box, in one side of which is to be fixed 

 a tube, having a convex lens in it : within the box is a plane 

 mirror, reclining backwards from the tube, in an angle of 

 forty-five degrees. The picture is formed on a square of un- 

 polished glass at the top of the box. If a piece of oiled 

 paper be stretched on the glass, a landscape may be easily 

 copied ; or the outline may be sketched on the rough surface 

 of the glass. 



Questions. — 1. What is a Ions ? — its axis ? — focus ? 2. Describe 

 the five kinds of lenses. 3. What proportion is there between the 

 common heat of the sun and the heat of the focus of a double convex 

 lens ? 4. Describe the burning glass formed at London. 5. What ex- 

 amples are given of images of objects being inverted by a convex lens .-' 

 6. How may a camera obscura be made ? 7. Why is the mirror 

 placed at an angle of 45 degrees exactly ? Ans. To throw the image 

 on the top, for incident rays, falling upon a surface declining 45 de- 

 grees, will be reflected at an equal angle of 45 degrees. 8. Describo 

 figures 30. 36. 32. 33. 



mm 



LESSON 33. 



3Iirrors. 



Panoram'ic, exhibiting a succession of objects. 



Opti^cian, a maker of optical instruments, one skilled in optics. 



Mirrors are made of glass, silvered on one side, or of 

 some metal highly polished. There are three kinds of them, 

 the plane, the convex, and tlie concave. Objects seen in 

 convex mirrors are diminished. A globe of glass, silvered 



