Decembee 29, 1911] 



SCIENCE 



895 



of the telescope when used with a prism, 

 the latter must be so large that the light 

 which falls upon it entirely fills the object 

 glass. The efficiency of the prism then de- 

 pends on its size and on its dispersive 

 power. 



In order to form an idea of the sepa- 

 rating or resolving power in spectroscopic 

 observations it will be convenient to con- 

 sider the Fraunhofer line D of the solar 

 spectrum, or the brilliant yellow line corre- 

 sponding to the radiation given out by a 

 salted alcohol flame. This Fraunhofer 

 recognized as a double line, and the length 

 of the light-waves of the components are 

 approximately .0005890 mm. and .0005896 

 mm. respectively. The difference is then 

 6/5,893 of the whole, or about 1/1,000, re- 

 quiring a prism of resolving power of 

 1,000 to separate them. If the prism were 

 made of flint glass with a base of 25 mm. 

 it would just suffice to show that the line 

 was double. 



Now we know of groups of spectral lines 

 whose components are much closer than 

 those of sodium. For instance, the green 

 radiation emitted by incandescent mercury 

 vapor consists of at least six components, 

 some of which are only a hundredth of 

 this distance apart, and requiring therefore 

 a resolving power of 100,000 to separate 

 them. This means a glass prism of 100 

 inches, the construction of which would 

 present formidable difficulties. These may 

 be partially obviated by using twenty 

 prisms of 5 inches each; but owing to 

 optical imperfections of surfaces and of 

 the glass, as well as the necessary loss of 

 light by the twenty transmissions and forty 

 reflections, such a high resolving power 

 has not yet been realized. 



The parallelism of the problems which 

 are attacked in astronomy and in spectro- 

 scopy is illustrated in the following table. 

 It is interesting to observe how intimately 



these are connected and how their solution 

 depends on almost exactly the same kind 

 of improvement in the observing instru- 

 ments, particularly on their resolving 

 poiver; so that not only are the older prob- 

 lems facilitated and their solution corre- 

 spondingly accurate, but new problems be- 

 fore thought to be utterly beyond reach 

 are now the subject of daily investigation. 



Astronomical 



Spectroscopic 



1. Discovery of new Discovery of new ele- 

 stars, nebute and ments. 



comets. 



2. Star positions. Wave-length of spec- 



tral lines. 



3. Double stars and Double lines, groups 

 star clusters. and bands. 



4. Shape and size of Distribution of light in 

 planets and nebulae. spectral ' ' lines. ' ' 



? Star discs. 



5. Star motions (nor- Star motions (parallel 

 mal to line of with the line of 

 sight). Resolution sight). Resolution 

 of doubles, solar of doubles, solar 

 vortices, protuber- vortices, protuber- 

 ances, etc. ances, etc. 



6. Changes of character 



and position of 

 lines with tempera- 

 ture, pressure and 

 magnetic field. 



7. Speetroheliograph 

 (Combination of telescope and spectroscope.) 



We must especially note that the newer 

 problems require an enormous resolving 

 power. In the telescope this has been ac- 

 complished partly by the construction of 

 giant refractors and partly by enormous 

 reflectors; and curiously enough the same 

 double path is open to spectroscopy; for 

 we may employ the dispersive power of 

 refracting media or the diffractive power 

 of reflecting media. The increasing cost 

 and difficulty of producing large trans- 

 parent and homogeneous blocks of glass 

 have tended to limit the size and efficiency 

 of lenses and of prisms, and these have 



