160 Suggestions for Observation 



that it will be very desirable that each observer's attention 

 be confined to very few phenomena. No party, probably, 

 will be able to make all the observations mentioned below ; 

 it will be desirable, therefore, carefully to select those which 

 can be made with the greatest probability of success, and to 

 give the utmost attention to those only. 



13. A quarter of an hour before the first contact of the sun 

 and moon, the observations of radiation with the actinometer, 

 &c., should be begun. (These should be continued through 

 all stages of the eclipse.) The commencement of an eclipse 

 is a very indistinct phenomenon, and perhaps for the principal 

 part of the time before the total obscuration little can be 

 done except to make, from time to time, observations of ra- 

 diation and meteorological observations. But when the limb 

 of the moon crosses that of the sun at right angles, and after- 

 wards, the observers will be well able to estimate (as far as 

 <;an be done by the eye) the difference in the intensity of light 

 on different parts of the sun's disc. From this time also it 

 will be proper to examine whether the entire circumference 

 of the 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 probably 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 ob- 



