PHYSICS. 365 



2. Reflection and refraction. 



A uew method of reading small angular deflections, like those of gal- 

 vanometers, for example, has been de vised by D'Arsonval. It may be 

 briefly described as the inverse of Poggendorfl''8 (subjective) method. 

 Usually the objective of the observing telescope forms at the conjugate 

 focus a diminished image of the object — the scale as reflected in the 

 mirror. D'Arsonval places the scale — a small one reduced by i)hotog- 

 raphy, giving tenths and twentieths of a millimeter — at this conjugate 

 focus and obtains a magnified image of it reflected in the mirror and 

 situated above the objective. This enlarged image, which is enormously 

 displaced for small angular movements of the mirror, is again observed 

 by an eye-piece bearing the usual cross wires. (Nature, April, 1886, 

 XXXIII, 610.) 



. Mouchez has described a magnified form of mercury bath or artificial 

 horizon which was constructed by Gautier for the Paris Observatory. 

 A cylindrical cast-iron vessel, containing mercury, cairies an axis at its 

 center, on which is cut a screw-thread. A second and slightly smaller 

 cast-iron vessel, having a similar screw-thread tapped through the bot- 

 tom, is placed within the first, movable up and down on the screw- 

 thread along the axis. This inner vessel is perforated with a small 

 hole through which the mercury enters from the outer vessel to form 

 the reflecting layer when the inner vessel is screwed down. This re- 

 flecting surface is found to be independent of the vibrations of the earth, 

 but only under the condition that the screw be neither very tight nor 

 very loose; since in the former case the two vessels are too firmly 

 united and the vibrations are communicated from one to the other; 

 and in the latter the inner vessel simply floats on the mercury, appar- 

 ently assuming a condition of unstable equilibrium, the variations then 

 producing in the mercury slow wave motions, thus preventing observa- 

 tion. The use of this new apparatus has thus far given excellent re- 

 sults at the Observatory. (O. 11., January, 1886, cii, 147.) 



The results which have been obtained by Abbe, as the outcome of 

 several years of experimenting to produce new and better kinds of op- 

 tical glass,have now been published. These experiments began in Jan- 

 uary, 1881, and were prosecuted in connection with Schott, who took 

 the chemical part of the work. Abbe himself taking the optical. The 

 progress made justified the building a special laboratory in Jena in 

 1882, containing furnaces in which 10 kilograms of glass could be melted 

 at once. Two problems occupied atteution.duriug 1883. The first was 

 the production of pairs of kinds of flint and crown glass, such that the 

 dispersion in the various regions of the si)ectrum should be for each 

 pair as nearly as possible proportional. The second was the produc- 

 tion of a greater multiplicity in the gradations of optical glass in re- 

 spect to the two chief optical constants, refractive index and mean 

 dispersion. In the fall of 1883 the two problems'were regarded as sat- 



