A LIQUID CONCAVE MIRROR. 



By Clinton H. Hulbtjrt 1 and R. F. Bacon. 

 {From the Chemical Laboratory, Bureau of Science.) 



The recent publications of Wood 2 on liquid mirrors induces us to 

 describe a similar mirror with which one of us has been experimenting 

 for several years. 3 



The general idea of using centrifugal force on liquids to form perfect 

 paraboloid surfaces seems to be very old, but its practical application is 

 of quite recent date. 



The principal problem which we undertook to solve was largely a 

 mechanical one. To obtain a perfect mirror it is obviously necessary 

 that the vessel containing the liquid should rotate at a uniform speed 

 and without vibration. The apparatus which we finally evolved to meet 

 these conditions is shown in Plate I. The mirror proper is formed in the 

 innermost of three cylindrical copper vessels, 22, 30, and 36 centimeters 

 in diameter, respectively. The two smaller vessels have double sides 

 with an air space between the walls to give greater buoyancy. In most 

 of our experiments the liquid used was a heavy, black paraffine oil, and 

 each vessel was filled about one-quarter full with this oil. For most 

 purposes the paraffine oil works quite as Avell as mercury. The two inner 

 cylinders have at the center of the bottom, double cones of steel which 

 causes these vessels to be held in the exact center of the apparatus by 

 means of the magnetic solenoid A. To rotate the pans, an ordinary- 

 laboratory motor of one-eighth horsepower was used. This was equipped 

 with suitable gears to vary the speed of rotation and the power was 

 transmitted to the pans by means of a belt, the whole apparatus being 

 mounted on a heavy wooden board. As the oil used is quite viscous, 

 any vibrations due to mechanical imperfections in the bearings of the 

 largest pan are absorbed by the oil, and not communicated to the other 

 two pans. Similarly, any variations in the speed of the motor are 



1 Formerly in the Quartermasters Department, United States Army. 



2 Astro Physical Journal (1909), 29, 165. 



'Mr. Hulburt 4 first began this work in 1902 and the writer (Bacon) became 

 associated with him early in 1906. A note was published in the Scientific 

 American, 1907, p. 103. 



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