A Volcanic Phenomenon. 333 



with which many are already familiar. He trusts that this is 

 so and that the flashing ares have been observed, whether by 

 volcanologists or during experiments with high explosives. 

 The only difficulty in the way of their being produced artifi- 

 cially would seem to be that of giving to them a sufficient size 

 to ensure visibility. A charge of dynamite exploded in an 

 open mortar would imitate the volcanic conditions, but on a 

 small scale. 



As observed by the writer at Vesuvius and Etna, the flash- 

 ing arcs may be considered one of the most beautiful of all 

 volcanic phenomena. 



Naples, May 27, 1912. 



Art. XXXIII. — The Comparison of Tioo Screws /* by Carl 



Barus. 



1. Introductory. — In work where two lengths are to be 

 compared, a shift AJV measured with one micrometer, for 

 instance, and a thickness e with another (a caliper screw), it 

 becomes of importance to coordinate these data directly. This 

 may be done with facility by mounting the opaque mirrors M 

 and N of an interferometer of the displacement typef on the 

 two screws in question, shifting the ellipses away from the 

 spectrum line with the first screw at JV and returning them to 

 the identical line with the other screw at M, through successive 

 consecutive steps of their length. The screws must now be 

 identically reversed in relation to the fixed beam of light and a 

 similar series of complete data investigated. The true relation 

 is the geometric mean of the two results, at each step. 



2. Method. — In fig. 1 let M and N be the opaque mirrors, 

 actuated by the micrometer screws at an angle a and a' respec- 

 tively, to the beams of light m and n, i. e., the normals of the 

 mirrors. G is the collimator, G the grating, and T the tele- 

 scope. Let the latter be clamped, so that the direction T G is 

 fixed throughout the experiment, as is also C G. It must be 

 possible to remove the mirrors M and JV, together with their 

 micrometer screws, without changing the angles a and a' in 



*Eeprint abridged from a forthcoming Eeport to the Carnegie Institution 

 of Washington, D. C. 

 f This Journal, xxxiii, pp. 109-119, 1912. 



