RECENT ADVANCES IN SCIENCE 21 



of photographic magnitudes, of types of stellar spectra, of 

 radial velocities, which has led to the discovery of numerous 

 variable and binary stars, and which has enabled stellar paral- 

 laxes to be determined with ease and accuracy. It seems 

 probable that a new step forward will result from the applica- 

 tion of photoelectric phenomena. The principles of photo- 

 electric photometry have already been used by Guthnick in 

 Germany and by Stebbins in America for the study of variable 

 stars whose range of brightness is very small. A paper by 

 A. F. and F. A. Lindemann in the March number of the Monthly 

 Notices, R.A.S., giving an account of the various possible appli- 

 cations of the new method as well as of details of the neces- 

 sary apparatus should, therefore, be read with interest by all 

 who are interested in the matter. It is worth while remark- 

 ing that, whilst many of the most recent astronomical develop- 

 ments have been limited in their application to telescopes of 

 large aperture and having lenses or mirrors with the highest 

 perfection of figure, this is not the case with the new method ; 

 it does not necessitate a very good lens or mirror, nor even a 

 very accurate driving clock, and although large apertures are 

 advantageous, as they enable observations to be made more 

 rapidly, its use is not restricted to them. It is, therefore, emi- 

 nently suitable for application by amateur astronomers pos- 

 sessing telescopes with apertures of, say, 6 inches and upwards. 

 The principle of the method is that light falling upon a 

 metal surface causes it to emit electrons whose energy is pro- 

 portional to the difference between the frequency of the light 

 and a frequency characteristic of the metal. With most metals 

 the latter is large, and the current obtained is weak, but with 

 the alkali metals a well-marked selective effect is obtained in 

 the region of the visual spectrum. The important property 

 of the phenomena is that the emission current is proportional 

 simply to the amount of the incident light and is not dependent 

 upon its intensity per unit surface. The practical application 

 consists in placing the photoelectric metal close behind the 

 focus of the telescope, so that the light from the object under 

 observation falls upon the metal. The number of electrons 

 emitted is measured, and the ratio of this number to the 

 number emitted under the influence of a standard light deter- 

 mines the energy received. 



For astronomical applications it is necessary to use the 



