412 



Physics. — "On the passage of Hijht tln'oiitjli tin' slit of a spec- 

 troscope." B_v Pi'of. P. Zkeman. 



(Communicated in the meeting of June 26 1915). 



Tlioiigli (lie full resolving- power of a ,sj)ectroscope can lie reached 

 witli an infinitely narrow slit only, we can get a near approach to 

 it with a very narrow slit already. 



This is evident, when with Schustkr ') we introduce the so-called 



"normal" widlli of the slit (/ defined by the relaiion f/ ^ ^^ — , where 



/ is the focal distance of the collimator objective with the diameter 



D and -> the wavelength of the incident light. For an exceedingly 



narrow slit the "purity" ') of the spectrum becomes equal to the 



resolving power. For normal width the parity is only 1,4° , below 



its maximum value. With a slit of twice the normal width we get 



about the double quantity of light, while the purity deviates 5,7% 



only from the maximum value. More than four times the normal 



width of the slit must never be taken, for then the purity of the 



spectrum decreases rapidly, while even for an infinitely wide slit the 



intensity of the light never exceeds four times the value obtainable 



with normal width. 



Resuming, we may conclude that for sufficiently intense sources 



A 

 of light a width of slit in the neighbourhood of ^^ — is the best. A nar- 



rower slit causes loss of light without gain in resolving power and a 

 wider one already soon decreases the resolving power considerably. As 

 example I choose a collimator with a lens, for which Z)=15cm., 

 ƒ =325 cm., while ). may be 5 X lt)~* cm. Then we have 



f/ = — =0,0027 mm. 

 ill 



Some time ago') I pointed out, that by very narrow slits the 

 oiiservalioii of the polnrized conqionents of magnetically resolved 

 Hues may be rendered very difficult. When graduall^y the slit was 

 made narrower, the ^electric) vibrations that are perpendicular to 

 the length of the slit, are hardly transmitted at last. 



It seems inieresiino- to communicate some measurements concern- 



') A. Schuster The optics of the spectroscope Astrophys. Journ 21, 197. 1905. 



-I Schuster, f.c. See also eg Zeem.^.n. Researclies in magneto optics, p 7. London, 

 Mac-Miillan. 19i:^. 



■) "On the polarisation impressed upon light by traversing the slit of a spec- 

 troscope and some errors resulting liierefrom." These Proceedingsp. 599. October 1912. 



