1891.] Focometry of Lenses and Lens-Combinations. 225 



radiation ; but the electric energy may equally well be directly con- 

 verted into the motion of radiation. As a fact, we nave never yet 

 been able to obtain either the emission or the absorption spectrum of 

 hydrogen without the aid of an electric current, so that, in reasoning 

 on this spectrum, we are much more in a region of speculation than 

 when treating of flames. Whether the hydrogen lines, bright or 

 dark, in the solar spectr,un are produced directly by the high 

 temperature of the sun, may even be called in question. And though 

 we may admit that the density of the hydrogen in the sun's atmo- 

 sphere, outside the photosphere, is but slight, it does not follow that 

 the total pressure of all the gases forming that atmosphere is so very 

 .small as Messrs. Frankland and Lockyer (' Boy. Soc. Proc.,' vol. 17, 

 p. 288) have, from the width of the lines, concluded it to be. After 

 all, it is not so easy to connect the temperature, even of a flame, 

 with its radiation, for it is only when the condition of a gas is steady 

 that we can assume that there is a definite relation between the 

 motion of agitation, on which temperature depends, and the vibratory 

 motions, on which radiation depends. In speculating on such 

 questions, chemical, as well as electrical, changes must not be lost 

 sight of, although the latter may be more -directly concerned in 

 radiation. 



Experiments which we have commenced upon the arc in an 

 atmosphere of compressed gas tend to the same conclusion. It does 

 not appear that the metallic lines in the arc are sensibly affected by 

 a steady pressure up to 15 atmospheres. The details of these observa- 

 tions, which are complicated by the variation of resistance with 

 ige of pressure, we -defer until the experiments are finished. 



[I. " On the Focometry o'f Lenses and Lens-Combinations, and 

 on a new Focometer." By SILVANUS P. THOMPSON, D.Sc., 

 B.A., Professor of Physics in the City and Guilds Technical 

 College, Fiiisbury. Communicated by Professor G. CAREY 

 FOSTER, B.A., B.Sc., F.R.S. Received February 4, 1891. 



(Abstract.) 



Few of the accepted methods of focometry take into account the 

 stance between the two principal points (or Gauss points) of a lens, 



afford the means of measuring this distance, as well as the true. 



il length, and some of them are open to the objection that they 

 jessitate troublesome double adjustments. Of these methods the 

 ithor gives a brief categorical review. 

 He has devised a method in which there are no double adjustments, 



measurements of the size of optical images, no assumptions as to 



