LXXXVI. NEW SOUTH WALES LIGHTHOUSES. 



Steam Navigation Board: — E. 0. Moriarty, Chairman; B. Darley, 

 C. Smith, J. Watson and H. T. Fox. 



The lighthouses are now under the jurisdiction of the Marine 

 Board appointed under the Navigation Acts of 1871-96. 



Lighthouse Optics. 



Before describing the lighthouses it will be desirable to re-state 

 a few of the principles of lighthouse optics. 



Three laws govern the change of direction in the incident 

 light produced by refraction — 



1. Incidence and refraction in a structure such as glass occur 

 in a plane perpendicular to the refracting surface. 



2. The sines of the angles of incidence and refraction have a 

 fixed ratio, called the index of refraction, and a ray of light 

 falling normally on a surface suffers no refraction. 



3. The effect on a ray of light passing from air into glass is to 

 make the angle of refraction less than the angle of incidence, and 

 the converse takes place on passing from glass into air. 



A ray of light passing from glass into air has its angle of re- 

 fraction greater than its angle of incidence, and there is some 

 angle of incidence whose angle of refraction is greater than a 

 right angle. Beyond this no refraction can take place, and a ray 

 is totally reflected. 



The index of refraction of glass is about § . This index is 



usually represented by the letter /x, and is equal to the 



sine of the angle of refraction divided by the sine of the angle of 



sin t 

 incidence = — — -. If, therefore, the incident ray makes a 



sin t ' ' J 



greater angle than about 42° with the normal, total reflection 

 takes place. A ray of light passing through a plate of glass with 

 parallel surfaces emerges parallel to its original path, suffering 

 displacement only. 



When a ray passes through a triangular prism it is bent 

 towards the base of the prism. 



The piano concave, double concave and double convex lenses 

 need not here be considered, the piano convex being the one 

 almost universally used in lighthouse apparatus. 



