154 



OPTICS. 



Lesson XI. 



we will try two si-uple experiments. [Experiment 20 ] I have placed a shilling at 

 the bottom of this teacup, and if 3011 walk back, so that you can merely see the edge 

 of it, I shall be ready to perform the experiment. [The pupil does so.] I will pour 

 some water into the teacup, and the shilling will then appear to rise more and more, 

 as the level of the water rises in the vessel, until at bst the whole piece of money will 

 become visible, because the refracting power of the water is gr< -ater than that of the 

 air. [Performs the experiment.] The next experiment is still more simple. [Ex- 

 periment 21.] Here is a tumbler of water and a spoon; now you will observe, when I 

 place the spo >n in the water, that it will appear as if it Mas bent. [Does so, and the 

 spoon appears bent.] This is because the light reflected from the spoon is refracted 

 as it issues from the water. An oar, a stick, or any straight object, will produce the 



same effect. I will illustrate the atmo- 

 spheric refraction of light by a simple 

 diagram (Fig. 36). Here we have four 

 lines, A A, B B, c c, and D D, parallel to 

 one another ; let these represent the 

 strata of the atm< sphere, which become 

 denser as they approach the surface of 

 the earth ; s represents a star, from 

 which a ray of light proceeds in a straight 

 line s a, until it enters the atmosphere at 

 a, its direction is then changed into the 

 fi 9 3 C line a b, then when it reaches the line 



B B into b c, and again when it reaches the line c c into c, finally reaching the eye at 

 o. The effect of this would be to make the star appear at s', which therefore raises 

 it from its true position by refraction, the same as the shilling in the teacup. 



QUESTIONS. 



127. T. Are rays of light always 

 changed in their direction when passing 

 from a rarer into a denser medium? 



p. _ No. If 

 they fall in the 

 u / direction of the 

 perpendicular r 

 !', Fig. 37, their 

 direction is un- 

 changed, and 

 there is no re- 



p r fraction ; but if 



fig. 37. the rays fall 



obliquely as R, they are bent towards the 

 perpendicular. The opposite of this takes 

 place in every respect where the rays pass 

 from a denser to a rarer medium. 



128. T. If what you have stated 

 about the medium be correct, different 

 substances must posses different powers 

 of refraction. Is this the case ? 



P. Partially so. A solution of salt is 

 a more powerful refractor than pure water, 



oil of turpentine more refractive than salt 

 and water, and oil of aniseed even still 

 more powerful than the turpentine. 



129. T. How is this ? You have said 

 before that the denser a body is, the 

 greater is its refractive power. 



7>. It has been found that coinbustible 

 bodies have greater refractive power than 

 incombustible substances of equal or 

 greater density. It was the knowledge of 

 this that led Sir Isaac Newton to believe, 

 that tin: ditmwnd was an inflammable sub- 

 stance. 



130. T. What substances are ge- 

 nerally employed by opticians for exhi- 

 biting the phenomena depending upon the 

 refraction of light ? 



P. Glass and transparent crystals, but 

 chiefly the former. 



131. T. Does the form of the refrac- 

 tive body exercise any influence upon the 

 relation of the refracted rays? 



/>_y ( . s . |>,y varying the obliquity of 



