SUGGESTIONS TO ASTRONOMERS 371 
the circle ; and let this be carefully verified once at*least. Let him 
note whether any kind of arch of light connects two or more of the 
-prominences. 
4. Let the dimensions and forms of the prominences be studied. 
For the former purpose reference should be made to the two parallel 
threads in the eye-piece of the telescope. For the latter, observe— 
Whether the prominences have hard and permanent, or waving and 
ill-defined outlines. Whether they are imvariably broadest at the 
base, and have on the whole a tapering shape. Whether they seem 
to stand erect, or whether any or all of them are aslant, like teeth 
on the edge of a circular saw. Whether any of them taper mwards 
next the dark limb of the moon: whether they appear isolated ; 
and, if so, how the space between the red patch and the moon’s 
limb is occupied. Whether the prominences vary in outline during 
the scrutiny. Whether any appear to grow up or to diminish ; 
and, if so, whether such change is what the moon’s motion ‘would . 
naturally account for. 
5. Let the zl/umination of the prominences be studied. First, as 
to general colour ; by inspecting them with the undefended eye, 
both with and without a telescope (without any dark glass). Next, 
as to distribution of colour, select a well-defined promimence and 
examine it all over repeatedly with a considerable magnifying 
power, and observe if it appears absolutely uniform in colour and 
brightness, or whether it shows any marks of structure or shadow 
or variation of tint. It seems very difficult to suggest any com- 
parative experiment for recording the brightness of the illumina- 
tion of the prominences. 
6. As the total phase goes off, let the eye be fixed on one or 
more of the prominences, and see whether they instantly and totally 
vanish, or for how many seconds they can be kept in view. 
᾿ It may be well to refer to M. Arago’s narrative of what was seen 
in 1842 and on former occasions, in the Annuaire du Bureau des 
Longitudes for 1846, and to a Beet by M. Faye in the eae 
Rendus de P Académie, 1850, Nov. 4 
Apprenpix No. III. 
Allusion having been made to instruments for determining the 
plane of polarization, it may be proper to give the following infor- 
mation :— 
Nicol’s prism is described in the Edinburgh Philosophical Journal, 
vol. xx. p. 83, and in the Philosophical Magazine, vol. iv. p. 289 ; 
and instruments on this construction are sold by Soleil in Paris 
᾿ς and Watkins and Hill in London. 
Savart’s polariscope is described in Peclet’s Traité de Physique, 
and is sold by the same artists. 
