Polytechnic Association: 671 



XIII. Proposed Improvement in Telescopes. 



The late Rev. "W. Y. Harcourt of England made a protracted series 

 of experiments, commencing as early as 1862, for the purpose of 

 reducing or eliminating the secondary spectra found in the best achro- 

 matic object glasses. It can be shown, theoretically, that any three 

 different kinds of glass may be made to form a combination achromatic 

 to secondary as well as primary spectra, but the actual construction 

 of such a triplet is beset with difficulties. Mr. Harcourt found that 

 by combining a concave lens made of glass containing terborate of 

 lead with positive lenses of ordinary glass, or else a positive lens con- 

 taining titanium with negatives of crown and flint, or a positive of 

 crown and negative of low flint, triple combinations free from second- 

 ary spectra might be formed, and that by substituting a borate of 

 lead for the flint glass, and a titanic glass for crown, still further 

 advantages might be gained. After encountering many difficulties, 

 discs of terborate of lead and of titanic glass, homogeneous throughout, 

 have at last been obtained, with which it is intended to construct an 

 object glass which shall be entirely free from secondary color. Such 

 an achievement will be a great advance in the optical art. 



jt XIY. Action or Magnetism on Gas. 



De La Rive and Sarasin have recently ^aid before a scientific society 

 of Geneva, Switzerland, an important communication, in which they 

 arrive at the following conclusions: 1. The action of magnetism, 

 when only exercised upon a part of an electric jet, projected through 

 a rarefied gas, causes an increase of density. 2. The same action on 

 an electric jet placed equatorially between the poles of an electro- 

 magnet produces in a rarefied gas an increase of resistance, which is 

 greater in proportion to the conductivity of the gas. 3. The same 

 action creates a decrease of resistance when the jet is directed in the 

 line of the axis between the magnetic poles. 4. "When the magnetic 

 action consists in the impression of a continued movement or rotation 

 of the electric jet, this action has no influence upon the resistance, if 

 the rotation is in a plane perpendicular to the axis of the cylinder of 

 magnetized iron which determines the rotation ; whereas the resist- 

 ance decreases considerably if the rotation takes place so that ^the 

 electric jet describes a cylinder of revolution about the axis. 5. 

 These several effects cannot be attributed to a variation of density 

 produced by the action of magnetism on the gas, but probably they 

 are explained by the perturbations that such action causes in the mole- 



