86 THE CHEMISTRY OF THE SUN. [CHAP. 



The advantage of the grating over the prism lies in the fact 

 that it gives a purer spectrum, for the light has no absorptive 

 material to pass through, such as prisms, the glass or surfaces 

 of which may be imperfect. 



But there is another and much more powerful reason which 



o 



induced Angstrom, Mascart, and many others who might be 

 named, to employ diffraction in preference to refraction. 



By the use of these diffraction gratings measured with great 

 care and expressed in terms of the standard metre, along with a 

 collimator and observing- telescope the latter fitted with a 

 micrometer screw which enables us to determine with great 

 accuracy the angle through which it moves we are able to 

 determine, in the most convenient manner and with great exact- 

 ness, the wave-length of the light we are observing. In this 

 way Angstrom determined the wave-length of the more pro- 

 minent lines of the solar spectrum from A to H. 1 Using these 

 lines as starting-points he was able, by means of the micro- 

 meter, to measure the angular distance between any of these 

 points and any line which lay between them, and then, writing 

 these determinations in interpolation formulae, he was able to 

 compute the wave-length of any observed solar line. 



The wave-lengths of some of the principal Fraunhofer lines 

 as given by Angstrom are as follows : 



A. -00076009 E -00052689 



B -00068668 F -00048606 



C -00065618 G -00043072 



D 1 -00058950 H -00039680 



D" -00058890 K -00039328 



1 Knowing the velocity of light, the wave-length of any particular line can be 

 calculated by the following formula : 



n 

 when A. = the wave-length. 



d = distance between lines of grating in millimetres. 



8 = observed angle. 



n =; order of the spectrum. 



