SOLAK ECLIPSE, MAY 0, 1883. 113 



Fyeks (]>. 107) observed with an iiitegTiitiiig spectroscope of about 1% of an inch aperture and 

 a single (iO^ ])risni. The bright lines C, 1);;, 1474 K, and K were seen. 



Ferguson (p. 470), with au apparatus similar to the hust, saw the same Hues as did Fyers, with 

 four other faint ones, the positions of which were merely estimated. 



The following observers employed analyzing spectroscopes : 



LocKYEK (p. 44ti) with a (W° prism of 14 inches eflective aperture sa\v a vivid hydrogen si)ec- 

 trum and the 1474 K line. He " was astonished at the vividness of the line and of the continu- 

 ous spectrum." 



Macleab (p. 447) with a '• Gprism spectroscope of great dispersive power" and a " 7-prism 

 direct-vision spectroscope," saw nothing whatever. 



Jassen (p. 4.50) fails to describe the essential constants of his apparatus, but emiihasizes the 

 tact that the image lens was of large angular aperture, thus, in the opinion of the observer, yield- 

 ing exceptionally brilliant spectra. We may be permitted to discuss this apparatus somewhat 

 minutely, not only as a good illustration of the theory given above, but on account of the impor- 

 tance of the observations made with it. 



A silvered mirror of 15 inches aperture and 60 inches focus was used as an " image lens." 

 The dispersive member consisted of two direct-visiouprisms. No telescope was used.* Nothing 

 is said about the aperture of the collimator and prisms except that they were so constructed as 

 to transmit the whole pencil from the mirror. Let us suppose the collimator 4 intihes in length, 

 which would correspond to an effective aperture of 1 inch. Then we should have a beam of light 

 1 inch in diameter from the prisms, but only that portion falls upon the retina which can pass 

 through the pnpillar aperture. If we suppose the diameter of the pupil to be J of an inch, then all 

 the light which reaches the retina comes from a portion of the image mirror 2 inches in diameter, 

 and the remainder is useless. Again, since such prisms are usually equivalent to two or three 

 prisms of 60°, we see that the apparatus is inferior to a single-i^rism spectroscope of f of au inch 

 eft'ective aperture even for observing bright lines; and, in accoi dance with the principles exi^lained 

 in the theory of the instrument, it is greatly inferior to such a single-prism spectroscope for detect- 

 ing dark lines. It is clear that any other assumption as regards the focal length of the collimator 

 would not affect the conclusion ; for, suppose the length twice as great, then, since the effective 

 aperture of 1 inch is not altered, only one- fourth as much of the mirror would be utilized; but, at 

 the same time, a slit of the same angular dimensions would have four times the area; thus an 

 exact compensation would i-esult.t 



The observer saw the hydrogen lines bright, as well as 1474 K. Besides these, D was seen 

 as a (larii line, with some faint ones in the green. The bright lines were traced to a distance of 

 10' to 12' from the limb of the moon. 



Herschel and Tennant (p. 454) give no description of the spectroscope used. They saw 

 1474 K only, and saw it at a distance of 8' from the moon. 



Moseley (p. 471). Captain Tupman describes the spectroscope used as having a collimator 5 

 inches focal length, 0.7 inch aperture, with a direct-vision prism of 2 flint and 3 crown prisms. 

 The image lens was of 3i inches aperture and 33 inches focus. " Tower of the whole combination 

 about 40." This gives effective aperture of 0.53 inch and magnifying power of 6. This should 



•• This fact was overlooked by Mr. Eanyard in liis discussiou of the same instruiuent. 



tMr. Kanyard, at p. 4.51, calls attention to M. Jannskn's fallacious reasouiu}; in a footnote. As, however, ho 

 not only conchules that tliis sjiecial apparatus was after all very efficacious, but also computes the "intensity of the 

 inuige" (pp. 375-378) for all the instruments used, we are driven to the conclusion that he overlooked the full force 

 of his logic. lu this connection his remarks on p. 37.') are very instructive. 

 S. Mis. 110 15 



