LIGHTHOUSE ILL UMINA TfON. 



207 



shows the path of the rays and the form of the section. 

 The ray enters normally, and therefore without loss, it is 

 totally reflected at r and returns again without loss. 

 There is a circle whose radius is the lamp f; where para- 

 bolas whose common focus is F ought to be used. They 

 are commonly made circles, osculating the parabola. The 

 whole section is then made to turn round a vertical axis. 

 The rings are now made separate from each other, a form 

 adopted by Mr. Chance of Birmingham, and approved of 



Fn. 2. 



by Mr. Stevenson. The great advantage in these mirrors 

 is in the diminished loss of light by reflection, the loss 

 being only about '230 in place of '444, the loss in metallic 

 mirrors. The apparatus you see here is what is called a 

 dioptric holophote. It is composed of one of these glass 

 totally reflecting mirrors, with an echelonned lens in front. 

 When in front of such an apparatus straight prisms are 

 placed of the form indicated in the section (Fig. 2), the 

 rays are all collected into foci, as at/'/', and then diverge 

 into an azimuthal angle due to the number of prisms, thus 

 the light will be seen in a vertical strip of the breadth of 



