288 REPORT — 1893. 



The nadirane now to be described was designed by M. Wolf for use 

 at tlie Pai-is Observatory.' It differs from that of M. d'Abbadie in two 

 important particulars: (1) The beam of light is made to traverse a 

 horizontal, instead of a vertical, layer of air, thereby avoiding the effects 

 of variations in temperature. (2) The relative fixity of the object-glass 

 and the observing microscope is rendered immaterial by employing 

 differential, instead of absolute, measures of the position of the image. 



The bath of mercary rests on the limestone floor of the observatory 

 cellar, which, being at a depth of 27 metres, is free from surface vibra- 

 tions, and remains at a practically constant temperature. Above the 

 bath, and rigidly connected with it, is a hollow prism of cast iron, the 

 hypotenuse face of which carries a plane mirror of silvered glass inclined 

 to the horizon at an angle of 45°. The mercury-bath is closed by a 

 horizontal object-glass, 24 cm. in diameter, about 80 m. in focal length, 

 and cut so as to give the least aberration to yellow light. The closing 

 of the bath was found to be necessary in order to prevent the action of 

 the mercury vapour on the silvering of the mirror. A second plane 

 mirror of silvered glass, and 14 cm. in diameter, is fixed to the 

 torizontal surface of a support rigidly connected with the framework 

 of the mercmy bath. On the floor of the cellar and in a line at right 

 angles to the vertical face of the hollow prism is a metal plate, through a 

 bole in which a beam of monochromatic yellow light passes. This beam 

 is reflected by the inclined mirror, and again by the horizontal mirror and 

 mercury, and is then returned as a double beam by the inclined mirror to 

 near its origin, where the two images are observed with a microscope 

 of small magnifying power. 



If the luminous point is rigidly connected with the optical centre of 

 the object-glass of the microscope the two images, M. Wolf shows, are 

 only displaced with respect to one another when the reflecting apparatus 

 is rotated about a horizontal axis. The luminous point and microscope 

 might indeed be held by the hand. The parts of the reflecting apparatus 

 must, however, be invariably fixed with respect to one another and their 

 support ; and, the temperature of the cellar being constant, it follows 

 that the only error can arise from rusting of the metallic pai'ts. 



1. ' Appareil destine ^ reconnaitre les Mouvements du Sol par la Variation de la 

 Pesanteur relativement aux Masses Solides du Terrain,' Paris, Acad. Sci., Com])t. 

 Bend., vol. xxxiv. 1852, pp. 942. 94H. 



2. 'Direction de la Pesanteur,' Paris, Acad. Sci., Coirypt. Rend., vol. Ixi. 1865, 

 p. 838. 



3. ' Etudes sur Ln Verticale.' Assoc. Fran^., Compt. Rend., vol. i. 1872, pp. 159-168. 



4. ' Observations relatives a une Communication de M. Plantamour sur le Deplace- 

 tnent de la Bulle des Niveaux a Bullc d'Air,' Paris, Acad. Sci., Compt. Rend., vol. 

 Ixxxvi. 1878, pp. 1.528-15.30. 



5. ' Sur les Variations de la Verticale,' Paris, Acad. Sci., Compt. Rend., vol. Ixxxix. 

 1879, pp. 1016, 1017. 



6. ' Recherches sur la Verticale,' Bruxelles, Soc. Scien., Annales, 5e ann6e, 1881, 

 pp. 37-51. 



7. ' Sur les Petits Tremblements de Terre,' Paris, Acad. Sci., Compt. Rend., vol 

 xcviii. 1884, pp. 322, 323. 



8. ' La Fluctuation des Latitudes Terrestres,' Rull. Astroji., mars 1892. An 

 abridged translation of this paper is given in Nature, vol. xlvi. 1892, pp. 65, 66. 



' ' Sur un Appareil propre il I'Etude des Mouvements du Sol,' Paris, Acad. Sci., 

 Comjyt. Rend., vol. xcvii. 1883, pp. 229-234. Notes on several changes in the 

 construction of this apparatus are given in the Rapports Anntiels mr VEtat de 

 rObserratoire dc Paris. 1881, pp. 20, 21; 1883, p. 14; 1884, pp. 20, 21; 1885, pp. 

 13, 14. 



