Dioptric Light erected at KMcald^, 253 



sui'face of which was covered with carefully worked putty ; the 

 same motions which had been used in smoothing, wete now em- 

 ployed in polishing, greater attention being paid to the fre- 

 quent reversion of the motions. 



It was on this combination of motions, which I relied for 

 producing a true finish. The nature of the action was this : 

 Suppose both motions to be dentral, the point of the polisher 

 would meet the surface of the glass obliquely, the minute 

 scratches inclining to the left, and the degree of inclination dc> 

 pending on the relation between the velocities. Let now the 

 motion of the lathe be reversed, the direction of the scratches 

 is immediately altered, and the one set of traces crosses the 

 other. By varying the velocities, the direction of the scratches 

 was still further changed, so that all those eflPects were, in this 

 way, produced, which are obtained from crossing the strokes in 

 the usual processes of grinding. 



The lens, after being finished, was supported on three brass 

 supports placed edgeways, so as to intercept little of the direct 

 light, and between these was placed a small argand gas-burner 

 with a sliding stalk, so as to be adjustable in height. The 

 lantern within which the whole is placed, is in the form of an 

 octagonal decaedron, its top and bottom being squares placed 

 45° upon each other, and its sides isosceles trigons. The form 

 was adopted because it contains theessential elements of strength, 

 and because the side astragals being all inclined, would not, 

 from any point of view, intercept a sensible portion of the light. 

 The support of the lantern is a cast from the pattern of our 

 })olice gas pillars ; it is imbedded deep in the mason work of 

 the pier. 



The entire apparatus has a very insignificant appearance, and 

 may readily be mistaken for a common street lamp. Notwith- 

 standing its exposed situation during the past nine months, it 

 has met with no accident (except the freezing of the water in 

 the meter), and has afforded a sufficient light at a very trifling 

 tost, and with scarcely any attendance; it is lighted in the eve- 

 ning and extinguished in the morning, and requires no atten- 

 dance in the interval. 



In designing this instrument it had to be kept in view, that 

 the light was not to serve for distant vessels, but merely for 



VOL. XXV. NO. L. OCTOBER 1838. S 



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