282 eeport— 1879. 



may be made apparent by the movement of a spot of light on a screen. It may 

 therefore in this respect be looked upon as a Thomson's Reflecting Galvanometer, 

 coiled with liquid instead of with metallic wires. The elements are placed one in 

 each of the little cells, and may be connected by a key ; or, by placing a reflecting 

 galvanometer in the external circuit, both currents may be simultaneously indicated 

 on the screen, and their interdependence or identity be demonstrated. 



The author is indebted to Mr. Gimingham, whose name is now inseparably con- 

 nected with the splendid researches of Mr. Orookes, for being able to produce the 

 instrument on the table, in which the tubes and coils are of glass, all in one piece, 

 and is a very beautiful specimen of accurate glass blowing. Below the base of 

 the instrument is a fine slightly magnetised sewing needle, which can be rotated 

 on a vertical axis through a small angle by means of a little lever, and by which 

 the instrument may be adjusted to zero. 



TUESDAY, AUGUST 26, 1879. 



The following Reports and Papers were read : — 



1. Report of the Committee on Astronomical Clocks. 

 See Reports, p. 131. 



2. Report of the Committee on RocJc-concluctivities. 

 See Reports, p. 58. 



3. Report of the Committee on Instruments for detecting Fire-damp in 

 Mines. — See Reports, p. 131. 



4. Suite des Recherches sur la Photograpliie Solaire. 

 Par Dr. J. Janssen, de VInstitut de France. 



La nouvelle methode est fondee sur trois conditions — 



1. L'achromatisme chimique de l'objectif, qui est fonde sur le maximum 

 d'action dans le spectre photographique. 



2. L'extension de la grandeur des images qui ont e"te" porters successivement 

 a 20, 30, 50 centimetres de diam6"tre. 



3. Le temps de pose, qui a ete r6"duit jusqu'a, 3^ et quelquefois 5^5 de seconde. 

 Resultats. — Oes photographies ont montre" que les formes admises pour les 



granulations n etaient pas exactes. 



Les formes sont celles de nos nuages atmospheriques, sauf qu'au lieu de vapeur 

 d'eau ce sont des poussieres metalliques solides ou liquides qui forment les nuages 

 solaires. 



Les photographies ont niontre" a la surface du soleil l'existence du reseau photo- 

 graphique — c'est-a-dire, que la surface solaire est divisee en regions de calme et 

 d' activity relatives. 



Les dernieres etudes ont montre que les formes et la grandeur des polygones 

 du reseau photographique sont variables, ce qui montre que les Amissions gazeuses 

 du soleil sont soumises a des penodes, qui sont sans doute en rapport avec les. 

 periodes des taches. 



