of the Total Eclipse of July 28, 1851. 161 



scuration). The light should be analysed, in regard to its 

 chromatic composition, by the use of a prism, and special 

 record made as to whether any of the colours are unusually 

 vivid or deficient. The general state of the sky and atmo- 

 sphere should be carefully observed and fully recprded. 



15. As the totality approaches, the sextant observer may 

 measure the interval between the cusps ; and the telescope 

 observers should examine carefully the state of the moon's 

 limb as to roughness, particularly in the central part (which 

 will be the last to touch the sun's limb), and should carefully 

 remark whether the moon's limb can be seen beyond the sun's 

 limb. These observations should be made with rapid changes 

 of the dark glasses. At the very time of completion of the 

 obscuration, Eaily's beads should be looked for, and if possible 

 with change of the dark glasses, and with change of the aper- 

 ture of the object-glass, and perhaps by putting the telescope, 

 for a moment, out of focus. (See Appendix No. I.) It will 

 probably be best, for the relief of the eye, that the observers 

 should now and then quit the telescope for an instant. The 

 time of total obscuration is to be communicated to the chro- 

 nometer-bearer by a single syllable. It is to be remarked 

 that, even though the error of the chronometer be not known, 

 the accurate observation of the duration of the totality will 

 give valuable information as to the diameters of the sun and 

 moon, and as to the moon's latitude. 



16. The naked-eye observer, in the mean time, is to look at 

 the sun with a dark glass, occasionally changing the glass, 

 occasionally trying the polarization, occasionally relieving his 

 eye. He may also specially remark whether the colour of 

 objects appears to change, and whether the light in different 

 parts of the sky is differently coloured. But when the totality 

 is near, he is simply to observe, ^ ith the weakest of the dark 

 glasses or (if possible) with the eye uncovered, and to note 

 the way in which the light is distributed, as to intensity, in 

 different directions round the sun ; whether there are beams 

 of light, and in what direction ; whether there are the rudi- 

 ments of a ring round the moon ; whether there is any light 

 on the side opposite the bright lune. It is recommended that 

 he do not quit this observation for any other ; but if a trust- 



VOL. LI. NO. CI. — JULY 1851. L 



