1861.] on De la Rue*s Photographic Eclipse Results. 363 



happened that under unusually favourable circumstances there occurred 

 a total solar eclipse ; the sun was nearly at his greatest possible dis- 

 tance from us, and therefore had almost a minimum apparent diameter, 

 and the moon was nearly at her least possible distance, and therefore 

 had a nearly maximum apparent diameter, so that the breadth and 

 duration of the total eclipse were nearly at a maximum ; moreover, 

 the shadow of the moon passed over a country easily accessible to 

 European astronomers. The speaker, after pointing out the course of 

 the eclipse, from sun rising in Greenland, across the Atlantic, across 

 Spain, the Mediterranean, and Africa, stated that for a belt of 60 miles 

 broad, the duration of the eclipse in Spain was fully three minutes, 

 and about three minutes and a half in the central line of that belt. 

 Such favourable circumstances were not likely to occur again within 

 the probable lifetime or opportunities of the observers, who went out 

 to Spain to observe the eclipse. Hence the desirability of placing 

 before scientific men the means used by various persons to record 

 and measure the appearances visible at total eclipses, with a view to 

 facilitate the labours of future observers. 



Curious and paradoxical as it might at first appear, it is necessary 

 to shut out the sun in order to see it ; for example, said the speaker, 

 look on this electric light, could any one conjecture from its aspect 

 what it is ? One sees a brilliant centre surrounded by rays, but one 

 cannot see the two charcoal points which are producing it ; and if it 

 were magnified and projected against a screen by means of lenses, 

 although the ignited points would be rendered evident, still there are 

 matters about them which would remain invisible. So it is with the 

 sun ; one can so reduce his light, by means of dark glasses, or other 

 contrivances, as to enable us to scrutinize by means of telescopes his 

 photosphere ; or we might project his image on to a screen, and thus 

 examine it : but we should not see the sun, that is the whole sun. His 

 mottled surface, his bright markings, his dark spots would undoubtedly 

 be thus shown ; but we should fail to discern those curious appen- 

 dages which were first observed a few years back on the occasion of a 

 total eclipse. These so-called red flames, about the nature of which 

 many conjectures have been made, are, as it now appears, true 

 belongings of the sun, and are not subjective phenomena, produced, 

 as some philosophers suppose, by a deflection or diffraction of the sun's 

 light in passing through the valleys on the moon's profile. 



The object of the speaker being, as before stated, to bring under 

 the notice of the Members of the Institution, Mr. De la Rue's photo- 

 graphic results : he now described the Kew Photoheliograph. A 

 photograph of the instrument and temporary observatory, taken in 

 Spain, being projected on the screen by the electric lamp. The helio- 

 graph consists of a telescope, the tube of which is square in section, 

 and which can be made to follow the sun accurately by means of clock- 

 work. The optical part consists of an object glass (actinically cor- 

 rected), and a secondary combination of lenses, situated near the focus 

 of the object glass, for the purpose of magnifying the sun's image to 



