160 LIGHTHOUSE ILLUMINATION. 



tion ; and you see above a still better arrange- 

 ment for the same purpose, one which comes 

 up to the maximum, I may say, of the ability 

 of directing light by this means. You are aware 

 that without that arrangement of glass the light 

 would be dispersed in all directions, but the lens 

 being there, all the light which passes through 

 it is thrown into parallel beams and cast hori- 

 zontally along. There is consequently no loss 

 of light, the beam goes forward of the same 

 dimensions, and will consequently continue to 

 go forward for five or ten miles, or so long as the 

 imperfection of the atmosphere does not absorb 

 it ; and see ! What a glorious power that is, to 

 be able to convert what was just now darkness 

 on that paper into brilliant light. 



Whenever we have refraction of this sort we 

 are liable to an evil consequent upon the ne- 

 cessary imperfections in the form of the lens ; 

 and Dr. Tyndall will take this lens, and will 

 show you even in this small and perfect appara- 

 tus what is the evil of spherical aberration with 

 which we have to fight. This can be illustrated 

 by means of the electric lamp ; if you look at 

 the screen, you will see produced, by means of 

 this lens, a figure of the coal points. This 



