62 Dr BREWSTEB on the Construction of Polyzonal Lenses, 



prism, as shewn in Plate VI. Fig. 1 . (of which Fig. 2. is a sec- 

 tion) where ABB'A', CD D'C', EFF'E', GHH'G', are the 



larger lenses, having the form shewn in Fig. 3. Plate II., and 

 having equal segments on each side of the centre, cut off by 

 vertical lines AA', BB', &c. The other lenses BC, DE, FG, and 

 HA, have the same height, but less width, and consequently 

 must be ground to a longer focal length than the others, in order 

 to be placed on the faces of the same prism. 



When the lampL, Fig. 2., is placed in the centre of this octohe- 

 dral prism, the whole zone of light which is contained between the 

 upper and under edges of the prismatic faces, will be concentrat- 

 ed into eight parallel and horizontal columns of light, every alter- 

 nate column having a different intensity. If the whole now revolves 

 in four minutes, we shall have a bright flame from the large 

 lenses recurring every minute, and a fainter one from the small- 

 er lenses every minute, so that there will be a reappearance of 

 the light every thirty seconds, and an eclipse every thirty se- 

 conds. By removing one or more of the lenses, variations in 

 the character of the light may be introduced to a considerable 

 extent. 



The advantages of the preceding construction may be thus 

 enumerated. 



1 . The whole zone of light which flows from the lamp be- 



tween the terminal edges of the prism is rendered 

 available. 



2. The lenses may be much more easily, and accurately fit- 



ted up, in the form shewn at AB B'A', Fig. 1. Plate VI. 

 than if they had a circular or a square form, as the edges 

 of the segments may be fitted into grooves in the ver- 

 tical bars A A', B B", and easily adjusted. 



3. Though, for a burning instrument the horizontal sides of 



a lens, which are cut off in Fig. 1., are as useful as 



