aOLAli ECLIPSE, MAY (i, 188^!. Ill 



iijioii tbe viilue of largo iiiigultir aperture in the image lens, in their published prograninies for 

 e(;Iii)se observations, l)ut guided by this erroneous theory they have too often failed to make the 

 most of rare opportuuities. This is a misfortune which would correct itself in time, but, unfor- 

 tunately, the time for such observations is restricted. Moreover, the results derived from elaborate 

 apparatus, even if based upon false theory, are apt to have undue weight accorded to them. 



In our examination of the existing spectroscopic evidence relating to the corona we may group 

 the observations, first, according to the date of the eclipse to which tliey pertain, and secondly, 

 according to the character of the instrument used, taking in order the prismatic telescope, inte- 

 grating spectroscope, and analyzing telescope. 



18C8. 



Of the four observers who directed their attention to the spectrum of the corona not one gives 

 the essential elements of the apparatus used. 



Ehia used a " Bunsen's spectral apparatus of the simplest construction" with a single prism. 

 The magnifying power of 20 was certainly far too high. Only a continuous spectrum was recog- 

 nized ; but the observer was confident that he would have seen dark lines in a solar spectrum of 

 the same brightness. 



Tennant's spectroscope is described as having a collimator of " short focus and considerable 

 aperture." As his image lens had a focus of 5 feet and an aperture of 4.0 inches, the effective 

 aperture of his spectroscope was probably very small. The dispersion was produced by a single 

 prism. Faint continuous spectrum observed. 



PoGSON observed faint continuous spectrum. Instrument not described. 



Eayet gives us sufticient data to enable us to make a fairly probable guess as to the dimen- 

 sions of his spectroscope. Por an image lens he used a reflecting telescoiie. His spectroscope 

 was a direct-vision instrument 40 cm. in length with collimator lens so calculated as to receive all 

 the light transmitted by the slit. If we suppose the reflecting telescope had an angular aperture 

 of about -^ this would imply an effective aperture of not far from 4 inch. As the direct-vision 

 prism probably had as large a dispersive power as two ordinary 00° prisms, we must conclude 

 that Eayet employed a very efficacious instrument. He observed three bright lines in the corona 

 spectrum which were doubttess D3 1474 K and F. 



1869. 



Pickering used an integrating spectroscope with angular a2)erture of 7°. Two bright lines 

 were seen, one near C and the other near E. 



Haekness gives the constants of his apparatus with great exactness, and, as he failed to get 

 the spectrum of the corona at first, but after widening the slit saw the coronal line with sufficient 

 distinctness to determine its position on the illuminated scale, his experience affords a valuable 

 guide in interpreting other records. According to his data the effective aperture of his single 

 prism spectroscope was 0.45 inch with a magnifying power of 5.7. This tailed to show anything 

 of the corona spectrum with a slit-width which had been used in recording the spectra of three 

 prominences. We may then fairly conclude that a spectroscope of less than i inch effective aper- 

 ture is ill suited for such observations. 



Young used an analyzing spectroscope with an effective aperture of ^V of an inch and a mag- 

 nifying j»ower of 18 ; but as it had five 45° prisms it was a very effective apparatus for bright lines. 

 "With it he determined with great accuracy the iiosition of the bright coronal green line, ^o dark 

 lines were seen though looked for. 



