366 REPORT—1850. 
moon, or any portion of it external to its intersection with the sun’s 
limb, can be seen. It may be necessary for this purpose to use a 
telescope with a small number of lenses, all the surfaces of which 
are well polished and perfectly clean. 
14, When the lune becomes narrow, occupying about a quadrant 
‘of the sun’s circumference, the state of polarization of the sun’s 
light in different parts of that quadrant may be examined. In 
these and subsequent observations of the same kind, it must be 
borne in mind that the diffused light reflected from air will pro- 
bably give traces of polarization, and it may be well in all cases to 
remark whether the brightest parts of the light under inspection 
are as evidently polarized as the faintest. Attempts should now be 
commenced for discerning whether a comparison can be instituted 
between the darkness of the shadows of a small object (as a pencil, 
or a small rod) formed by the sun and by the lighted candle; and 
whether the distance of the paper disc, when their shadows are 
equally black, can be ascertained. [If this is found practicable, this 
observation should be continued to and through the total obscura- 
tion.] 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 atmosphere should be carefully ob- 
served and fully recorded. 
15. As the totality approaches, the sextant observer may mea- 
sure the interval between the cusps; and the telescope observers 
should examine carefully the state of the moon’s limb as to rough- 
ness, 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 observa- 
tions should be made with rapid changes of the dark glasses. At 
the very time of completion of the obscuration, Baily’s beads should 
be looked for, and if possible with change of the dark glasses, and 
with change of the aperture 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 chronometer-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 valu- 
able 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, occa-~° 
sionally trying the polarization, occasionally relieving his eye. 
He may also specially remark whether the colour of objects ap- 
pears to change, and whether the light in different parts of the 
sky is differently coloured. But when the totality is near, he is 
