ON LIGHTHOUSE APPARATUS. 321 



their foci in the common focus of the apparatus, and their axes of 

 generation coincident with the horizontal axis. This is represented in 

 the model on the table. (Shown in fig. 5). It is quite plain that if this 



Figure 5. 



Section of one of the prisms of dioptric spherical m't/or showing path of ray of light. 



Instrument be properly constructed and for this purpose I may men- 

 tion that I was indebted to Professor Swan, of St. Andrews, for the 

 formula a person standing behind the apparatus will see no light, 

 although there is nothing between his eye and the intense flame which 

 is burning in the focus but a screen of transparent glass. You will 

 observe that in the focus of the apparatus on the table there is placed a 

 large red ball which you can see through the front half, but when I turn 

 the back of the apparatus to you, you will not be able to see it. You 

 will cee part of the stick that supports the ball because a large portion of 

 it is exfocal, but the focus, and whatever is near the focus, will be 

 totally reflected by this screen of transparent glass, and therefore you 

 are not able to see it at the back of the apparatus. 



I have to explain that since I designed this apparatus, which was 

 in 1850, my friend, Mr. James Chance, of Birmingham, suggested some 

 valuable improvements both as to the way of setting, and the mode 

 of generating the prisms. These totally reflecting spherical mirrors 

 Are now very generally used in lighthouses, and there is an example 

 of it down stairs to which I shall refer again presently. 



I have now described the means of producing the most intense single 

 beam of light from a single flame, but for the purpose of producing the 

 revolving light it is better to adopt the plan which Fresnel employed of 

 having one large flame surrounded by a framework containing large lenses. 

 This is the holophotal revolving light (fig. 6) with the great annular lens 

 of Fresnel, and holophotal prisms above and below, which intercept 

 the light in the way I have spoken of. There is a beautiful example of 

 this apparatus made by Messrs. Chance, exhibited by the Trinity House 

 in the yard outside. The frame revolves round about the light, and when- 



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