364 REPORT—1850. 
which the air is exhausted). In the selection of the instrument, it 
must be borne in mind that im Western Europe the sun will be 
high, and that the season of the year is (generally speaking) favour- 
able to energetic radiation; and also that it is desirable that the 
observation with the selected imstrument occupy as short time as 
possible. For meteorological observations, a dry thermometer and 
a thermometer with wet bulb (or other hygrometer) will probably 
suffice. 
8. It would be most desirable also to be furnished with some 
apparatus for measure of the intensity of light, but we are unable 
at present to particularize any which can be considered unobjection- 
able. The appearance of a lighted candle will give some very rude 
information. The flame of a candle may also be used for giving a 
good idea of the intensity of light, by viewing the object, whose 
brightness is to be ascertained, through the flame (thus, in ordimary 
sunlight, the light of the sky near the sun is seen through the flame 
without apparent diminution; but the light of a full moon cannot 
be seen through it at all). For the observation of shadows, a gra- 
duated scale, several feet long, with a disc of white paper to be slid 
upon it, with its plane perpendicular to the scale, may be useful. 
9. Some instrument should also be provided for ascertaining the 
state of polarization of the light. In the limited duration of a total 
eclipse, time is wanting for the use of instruments giving an accu- 
rate measure of the degree of polarization; for the rough estima- 
tion of the position of the plane of polarization, and of its general 
magnitude, perhaps a Nicol’s prism, furnished with plates of quartz 
cut in Savart’s manner, or a Savart’s polariscope, may be found 
convenient and sufficiently accurate. For the observation of the 
sun’s disc before the total darkness, a dark glass of some kind 
should be used with the Nicol’s prism. A common glass prism 
should be provided for examination of the chromatic composition 
of the light. 
10. At any fixed observatory within the path of the shadow, 
which is furnished with a telescope mounted equatorially, and 
moved by very good clock-work (adapted in its rate to the diurnal 
movement of the sun), it is extremely desirable that arrangements 
should be made for Daguerreotyping or Talbotyping the image of 
the sun, or of the light surrounding the moon when the sun is 
hidden. It is necessary to observe that materials of very different 
degrees of sensibility will be required in different stages of the 
eclipse ; the light of the uneclipsed sun being intensely bright, and 
that of the corona surrounding the moon, or even that of the red 
flames projecting into the corona, being exceedingly feeble. 
If the plate or paper be so sensitive to photogenic action that an 
image can be formed in a fraction of a second of time, no equa- 
toreal movement will be required. If an image can be formed in 
one or two seconds, a rude equatoreal motion, such as may be given 
to a temporary stand, will probably suffice. If this motion is given 
