158 Suggestions for Observation 



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 particularise any which can be con- 

 sidered unobjectionable. 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 ascer- 

 tained, through the flame (thus, in ordinary sunlight, the 

 light of the sky near the sun is seen through the flame with- 

 out apparent diminution ; but the light of a full moon cannot 

 be seen through it at all). For the observation of shadows, 

 a graduated 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 ascertain- 

 ing the state of polarization of the light. In the limited 

 duration of a total eclipse, time is wanting for the use of in- 

 struments giving an accurate measure of the degree of 

 polarization ; for the rough estimation 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 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 surround- 

 ing the moon when the sun is hidden. It is necessary to 

 observe that materials of very diff'erent degrees of sensibility 

 will be required in diflferent 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. 



