6*8 Dr BREWSTER on the Construction of Polyzonal Lenses, 



In the year 1820, I prepared a very thin slice of chalk, and 

 having exposed it to the heat of the blowpipe, I found that it 

 emitted a white and brilliant dazzling light, not much, if at all, 

 inferior to that which arises from the deflagration of charcoal by 

 the action of galvanism *. The idea afterwards occurred to Lieu- 

 tenant DRUMMQND of obtaining this intense light from a ball of 

 chalk a quarter of an inch in diameter, by directing upon it 

 three alcohol flames, by means of a stream of oxygen. The 

 light thus produced he found to be eighty-three times more in- 

 tense than the brightest part of the flame of an Argand burner. 

 Dr HOPE produced the same effect, by directing upon a ball of 

 lime the flames of oxygen and hydrogen proceeding from sepa- 

 rate vessels; and Dr TURNED, has accomplished the same object 

 by oxygen and compressed oil gas. 



In certain lighthouses, therefore, we would strongly recom- 

 mend such a light to be used, on great emergencies, when the 

 risk of human life, and of valuable property, would authorise 

 such an additional expenditure. 



VI. On the Introduction of Gas into JLic/hthous-es . 



Ever since the introduction of gas-light, its application to 

 the purposes of a lighthouse has been often suggested ; but 

 though the suggestion has been in some cases taken into conside- 

 ration, it has been invariably rejected, and there is not a light- 

 house under the superintendence of the English, the Scottish, 

 or the Irish boards, in which gas has been used, or in which 

 there is at present the slightest intention of using it f . 



* See Edinburgh Journal of Science, No. X. p. 139. 



f- Since writing the above, I have learned that gas has been used in one or more 

 lighthouses. 



