THEORY OF ETHER 171 



from its surface. If it be plane, the image remains clear and well 

 defined. 



4. Observe by sounding a tuning-fork, and slowly turning it when 

 held near the ear, that an interference of sound-waves, comparable with 

 the interference of ether-waves, may take place. 



5. Draw a diagram which will explain the formation of a spectrum 

 from a ray of white light on the hypothesis that waves of different 

 lengths are differently retarded in passing through a prism of dense 

 material. 



6. Explain why a parallel-sided transparent body merely changes 

 the path of a ray of light without decomposing it. 



7. What information as to the size of the ultimate particles of 

 matter can be gained from the undulatory theory of light ? 



8. Draw diagrams, and construct models with wires or suspended 

 bodies, to illustrate wave propagation and the interference of waves. 



9. The thermal focus coincides with the optical focus in the case of 

 a mirror but not in the case of a lens. Why is this ? Is the focal 

 length of a lens the same for all kinds of light ? 



10. Draw a diagram showing the course of light and the images 

 formed when a candle is placed between two plane mirrors inclined at 

 an angle. 



11. Why is the foam of clear water white ? 



12. Draw a diagram showing the method of obtaining an absorption 

 spectrum. 



13. Explain how a spectrum can be formed by an interference 

 grating. 



14. How can the colours visible upon a very thin soap-bubble be 

 explained ? 



15. What is known as to the rate at which light travels ? Distin- 

 guish between ' wave-length ' and ' period' as applied to light. 



16. Suggest an explanation of the various line spectra exhibited by 

 glowing vapours. 



17. What are the supposed properties of the ether ? 



18. It is said that all exact scientific knowledge is based upon the 

 measurement of matter and motion. Discuss this statement. 



