22 SCIENCE PROGRESS 



alkali metals, potassium, rubidium and caesium. These are 

 deposited on the walls of special glass cells containing a rare 

 gas. The method of construction of the cells is described in the 

 paper. Accurate methods of measurement are essential, and 

 for this purpose the authors constructed a specially sensitive 

 string galvanometer. A single observation can be made in 

 about four or five minutes, each observation consisting of a 

 reading of the electrometer deflection when the telescope is 

 set upon the stellar object, followed by a reading when a 

 standard voltage is switched on. 



The practical applications suggested in the paper are 

 numerous, (i) The determination of stellar magnitudes is 

 first referred to : the work of Guthnick and Stebbins has 

 already shown that a very high order of accuracy is obtain- 

 able, and that the variability of stars whose range of bright- 

 ness does not amount to more than a few hundredths of a, 

 magnitude can be established with certainty. Stebbins has 

 found the new method considerably preferable to the selenium 

 photometer for this purpose, and has abandoned the latter in 

 favour of photoelectric photometry. (2) The determination of 

 stellar temperatures is possible provided that a potassium 

 and a caesium cell are used alternately. These have maximum 

 sensitiveness at wave-lengths of 4,400 and 5,500 tenth-metres 

 respectively, and so the energies corresponding to two different 

 wave-lengths can be deduced. From this, the temperature 

 can be calculated on the assumption that Planck's radia- 

 tion law is obeyed. An investigation of the theory of the 

 method indicates that an error of 1 per cent, in the measure- 

 ment leads to an error of only about ioo° in the temperature. 

 (3) One of the most important features of the method is that 

 it enables an accurate measurement to be made of the light 

 received from extended surfaces such as nebulae or comets. 

 Thus the equivalent stellar magnitude of a nebula can be 

 determined, and, provided that its distance can be estimated, 

 an approximation can be made to the number ol luminous 

 stars which it contains. The temperature of nebulae can also 

 be determined. A knowledge of this is of importance on 

 account of its bearing upon the question of stellar evolution, 

 for, if the temperatures should be found to be approximately 

 the same, it would indicate that the life of the galaxy is great 

 compared with the light of a single star. Somewhat similar 



