INTRODUCTION. V 



is a similar section minus the adjoining wings and the red 

 media through which the white flash is transmitted. Both 

 flashes are intended to equal each other in intensity, which 

 accounts for the much smaller portion of the lens allotted 

 to the white. A belt of horizontal prisms, three feet wide, 

 connects the wings of the red section with the white on 

 either side, and reflects the rays of light equally to both 

 sections. Stooping underneath this belt, access is obtained 

 to a fixed circular platform in the interior of the lens. In 

 the centre of this platform, upon an iron pedestal, is placed 

 the source of illumination, a large six-wick lamp. Between 

 this platform and the top of the machine case, the circular 

 carriage upon which the superstructure of the lens is erected 

 revolves upon twelve five-inch steel rollers travelling upon 

 a circular metal pathway. Attached to the under side of 

 this carriage is a heavy gun-metal ring, six feet in diameter, 

 toothed on its inner circumference, which engages with a 

 horizontal pinion wheel rising from the corner of the machine 

 case, and through which the driving power is transmitted. 

 The machine an exaggerated form of clockwork is driven 

 by a weight of 400 Ibs. travelling in the centre of the spiral 

 stair on the first flat. The speed of the machine is regulated 

 by adjustable fans ; and a speed indicator, furnished with an 

 alarm bell, intimates the periods of winding an operation 

 necessitating two minutes' stiff winding every half-hour. The 

 entire lens, with its supporting carriage, is estimated to weigh 

 about six tons. The lamp, as I have already stated, is placed 

 upon an iron pedestal in the centre of the platform, or service 

 table, as it is called, in the interior of the lens. It is fitted 

 with what is known as the stepped Doty burner, and carries 

 six concentric wicks, each slightly elevated above the other 

 towards the centre. The burner is six inches in diameter, 

 and consumes paraffin oil at the rate of eighteen gills per 



