OPTICS. 



Lesson IX. 



QUESTIONS. 



105. 7'. What is light? 



P. Rapid undulations or vibrations 

 produced by very minute particles or 

 luminous bodies in a thin and elastic 

 medium, called the luminous ether, which 

 rposed between them and the seat 

 of our vision. 



IOG. T. How do these undulations 

 produce light ? 



P. By stimulating the optic nerve by 

 means of vibrations, and producing an 

 effect which we call light. 



107. 7'. How does light proceed? 



P. In a straight direc- 

 tion from the luminous 

 body which produces it, 

 towards the part upon 

 which it acts. Thus from 

 every luminous point rays 

 of light proceed in all di- 

 29. rections, as seen in this 

 diagram (Fig. 29). 



108. 7'. What do you mean by a ray 

 of light? 



P. It is the smallest portion of light 

 which can emanate from a luminous bod}-, 

 and is generally represented as a mathe- 

 matical line, although it is really an 

 infinitesimal pyramid. 



109. T. What do you mean by a 

 medium? 



P. -It is any transparent space through 

 which light passes, such as air, water, and 

 glass. Even empty space is a medium. 



110. T. What is a luminous body? 



P. It consists essentially of ponder- 

 able matter, and the ultimate physically 

 perceptible atoms are called luminous 

 points; therefore as every body is made 

 up of molecules or atoms, so i a lumin- 

 ous body made up of an assemblage 

 of luminous points. 



[The pupil should read Lesson III., p. 11.] 



111. T. Are all bodies luminous? 



p. No, some are opaque, as stones, 

 metals, wood and clay, and do not suffer 

 light to pass through them. This pro- 

 perty however depends upon their thick- 

 ness, for all bodies will admit of the 

 passage of some degree of light if we 

 make them sufficiently thin. For example, 

 if we affix a thin gold leaf to a glass plate, 

 and hold it to a strong light, we shall 

 perceive a blueish-green light through it. 



Others are transparent, as water, air, and 

 glass. These bodies yield a passage to 

 light, ?o as to allow us to observe the 

 form of objects beyond them. Translucent 

 bodies (such as thin paper and ground 

 glass), admit of the transmission of some 

 portion of light, without however allowing 

 the form or colour of objects being recog- 

 nised if they are far distant. I cannot 

 give you a more familiar illustration of 

 the comparative, but not the actual degrees 

 of distinction between opacity, transpa- 

 rency, and translucency, than the common 

 fowls' egg-shell. If you hold it when 

 boiled, and full agamst the light, you will 

 have a good definition of opacity. Now 

 empty it, and on holding it against the 

 light, you will observe that some parts of 

 the shell are darker than others, which 

 are spotted as it were with light, and that 

 there is also a thin membrane lining the 

 shell. Now the shell itself will represent 

 opacity, the light spots on it trauslucency, 

 and the membrane transparency. Be sure 

 to remember, however, that this is only 

 the comparative degrees of distinction, 

 because the whole of them are translucent. 



112. T. You mentioned before (Ques- 

 tion iqr>) that a ray of light proceeded in 

 a straight line towards the part upon 

 which it acts, now when it reaches that 

 part what takes place? 



P. As long as the ray remains in the 

 same medium it advances in a straight 

 line, hut as soon as it comes in contact 

 with another body it is partly thrown 

 back, or reflected from its surface : if the 

 body is transparent, the light partly enters 

 the body in an altered direction, and is 

 then refracted. 



1 13. T. Does light travel fast ? 



P. Yes, its velocity is so great, that it 

 traverses all distances upon earth in an 

 imperceptibly small space of time. It 

 travels over 195,000 English miles in one 

 second, and therefore would require eight 

 minutes and thirteen seconds to traverse 

 the space between the sun and the earth; 

 while a cannon-bull, going at the rate of 

 1,200 feet in a second, would require 

 fourteen years to pass through the same 

 space. 



114. T. How do you account for 

 shadows? 



