SOLAR OBSERVATORY. , 1 75 



Up to the present time it has been possible to make only a preliminary in- 

 vestigation of the four clusters for which Dr. Fath has obtained the class of 

 spectrum. These all yield results which are decidedly confirmatory of Pro- 

 fessor Kapteyn's expectations. The degree of redness as determined by the 

 difference of magnitude of an object as photographed (i) directly on a Seed 

 No. 27 plate and (2) through a Wallace red screen on a Seed No. 27 plate 

 sensitized for the red by Wallace's process is very different from what can 

 be accounted for simply by the class of spectrum. The differences, which 

 have as yet been only roughly estimated, are as follows : 



N. G. C. 7078 Cluster i.o mag. redder than star of same spectrum 



N. G. C. 7089 Cluster 0.4 mag. redder than star of same spectrum 



Hercules Cluster * 0.4 mag. redder than star of same spectrum 



Nucleus of Andromeda Nebula i.o mag. redder than star of same spectrum 



The importance of securing a method for the determination of stellar dis- 

 tances too great for a successful application of any existing method is ob- 

 vious. An extension of this work to other objects has therefore been 



planned.! 



Resolving Power of Gratings. 



During his visit to Mount Wilson Professor Chamberlain made some pre- 

 liminary tests of a device which he hopes may increase the resolving power of 

 gratings. The work will be continued by Professor Chamberlain at Vassar 

 College during the coming winter. 



PHYSICAL LABORATORY. 



The Pasadena laboratory, in which experimental work was beginning at 

 the time of the last report, has had its efficiency increased by many additional 

 fittings and by some new pieces of apparatus. The principal investigations 

 carried on during the past year have been : 



(i) Studies of the Zeeman effect. 



(2) Electric-furnace investigations, in which the effect of temperature as 

 a means of producing and modifying spectra was observed. 



(3) A study of banded spectra, as given by the vapor of calcium and other 

 substances when produced in an atmosphere of hydrogen. 



The observations in these several lines of work have their immediate ap- 

 plication in the study of the conditions prevailing in sun-spots, though the 

 results obtained from the electric furnace, in particular, are useful in the gen- 

 eral study of solar and stellar spectra and throw light besides on the tem- 

 perature conditions in the radiation of various laboratory sources. 



* Dr. Fath says that the spectrum is composite. The value here given assumes that 

 the spectrum of the cluster lies between F and K, which is probably a safe estimate. 



tit is not intended to deny a priori that causes other than space absorption might 

 possibly play a part in producing the observed differences. Thus, for instance, the 

 Andromeda Nebula may contain matter which scatters or absorbs the light coming 

 from the nucleus. Such points must be taken into consideration and means must be 

 devised to eliminate them from the problem. 



12 — YB 



