164 Suggestions for Observation 



all the incident light. Owing to its high refractive power, 

 flint glass is therefore more suitable than plate glass for the 

 construction of the totally reflecting mirror, as from their 

 greater breadth a smaller number of zones of the former 

 material will be required to complete a hemisphere, and the 

 expense of construction will accordingly be lessened. 



Suggestions to Astronomers for the Observation of the Total 

 Eclipse of the Sun, on July 28, 1851.* 



1. The principal observations for which a total eclipse of 

 the sun presents peculiarly favourable opportunities may be 

 classed under the following heads : — 



Observations applying specially to the physical structure 

 of the sun and moon, as those of the corona, and of the rose- 

 coloured prominences (seen so markedly in the eclipse of 

 1842.) 



Photometric, thermometric, and actino-chemical observa- 

 tions, illustrating the difference in the nature and amount of 

 radiation from different parts of the sun's disc. 



Optical observations, particularly on the state of polariza- 

 tion of the light in different phases of the eclipse, and on the 

 phenomena of irradiation, and of distortion of a dark limb 

 by the formation of beads or threads. 



The phenomena of the first class have never been seen ex- 

 cept in a total eclipse of the sun ; and they appear so de- 

 serving of attention (especially the red prominences, which, 

 if belonging to the sun, indicate physical peculiarities of 



* General Committee of the British Association for the Advancement of 

 Science. Edinburgh, August 7, 1850. Resolved, — That a Committee, con- 

 Bisting of Sir John Uerschel, the Astronomer-Koyal, Professor Forbes, and 

 Professor Powell, with power to add to their number, be empowered to commu- 

 nicate with the astronomers of Pulkowa on the observations to be made at the 

 next approaching Total Eclipse of the Sun, July 28, 1851 ; and to draw up 

 suggestions for the guidance of observers generally. 



[The above suggestions were drawn up by the Committee, with the assistance 

 of M. Otto Struve of Pulkowa. A copy of the " Suggestions " was kindly 

 transmitted to us, with permission to publish them, by the Astronomer Royal. 

 The map of the total eclipse could not be got ready in time,] 



