170 ELEMENTS OF LABORATORY WORK 



good reflexion grating with a heliostat exhibits these lines 

 very satisfactorily. 



The main result of our observations of the movements of 

 ether should be to impress upon us the need of considering it 

 as a possible agent in all our experiments. We have seen that 

 it is capable of handing on the state of one portion of matter 

 in one place to another portion in another place ; we have 

 seen that this is performed in a peculiarly subtle manner, and 

 we can never consider that any of our knowledge of natural 

 changes approaches completeness until we understand, not 

 only the changes of matter so called, but the changes of the 

 ether in its neighbourhood. In the future, the probable 

 ether movements corresponding with magnetic and electric 

 disturbance will have to be investigated, as well as further 

 details about the theory of light which may be gathered from 

 what is called the polarisation of light. We have to remember 

 that accuracy depends upon completeness of observation ; a 

 minute change, if unobserved, may overthrow an elaborate 

 structure of theory. At the same time we have to remember 

 that, the deeper observation goes, the more there is to 

 observe. 



Additional Exercises and Questions. 



1. Find the angle of a prism by reflexion, from each of its faces, of 

 light from a small body. The angle made by the two reflected rays will 

 be double the angle of the prism. Also find it by ascertaining the angle 

 through which the prism needs to be moved in order that a ray from a 

 small body may fall upon the two faces in succession and be reflected in 

 the same direction. The angle through which the prism has been moved, 

 together with the angle of the prism, should be equal to 180. Draw 

 diagrams in explanation of each process, and then use a spectroscope. 

 Preliminary observations may be made with a prism placed upon apiece 

 of paper, upon which lines may be ruled to coincide with the directions 

 of the rays and with the angle and movement of the prism. 



2. Produce a spectrum on a screen by use of a slit, lens, and prism, 

 and show that the colour of a body is not an inherent property by 

 placing it in different parts of the spectrum. 



3. Find out whether a reflecting surface be plane or curved by as- 

 certaining if it reflects regularly. Focus a telescope upon a small body, 

 then incline the surface so that a reflexion of the body may be seen 



