152 REPORT— 1859. 



tion to Spain, in July of next year, to obtain a photographic record of the 

 feeble light of the Corona and the Red Flames ; but it is by no means certain 

 that their light will be sufficiently intense for that object. Even a failure, 

 however, will prove of some value, for it will show that the image of these 

 phenomena, when enfeebled by an enlargement of eight times linear, possesses 

 too little actinic power to imprint their outline on a collodion-plate in a given 

 number of seconds; and thus data will be furnished for a future period. 



It is desirable that other astronomers should endeavour to obtain photo- 

 graphs of these data by placing the sensitized plate directly in the focus of 

 the telescope. 



In taking photographs with the Kew Photoheliograph, the telescope, 

 clamped in declination, is placed a little in advance of the sun, and then 

 clamped in right ascension ; the thread is set on fire as soon as the centre of 

 the sun coincides with the axis of the instrument. In order that the operator 

 may know when this is the case, a secondary camera or finder is fixed on 

 the top of the pyramidal tube of the telescope*. This finder consists of an 

 achromatic lens of long focus, which is so placed as to throw an image of the 

 sun on to a plate of brass fixed vertically near the lower or broad end of the 

 tube, and consequently in a convenient position for the operator to see both 

 the image and the retaining thread which holds the slide. The brass plate 

 has ruled on it several strong lines, two of which are just so far apart and so 

 situated as to form tangents to the sun's limb when the image is exactly 

 central; a lighted match, held in readiness, is at this precise moment applied to 

 the thread, and the slide immediately flashes across the secondary object-glass. 



Position Wires. — The position of the solar spots in respect to a normal 

 point is determined by placing a system of wires in a certain known position 

 in the telescope. Originally the wires were four in number, two being fixed 

 at right angles to the other two, the distance between each pair being some- 

 what less than the semidiameter of the sun ; so that when one wire of each 

 pair was situated near the sun's centre the other cut off a small arc at the 

 limb. The position of the wires was such that the one pair was parallel to 

 a circle of declination. 



Some inconvenience was occasionally experienced in consequence of one 

 or other of the four wires obliterating a solar spot ; hence an alteration is now 

 being made in the apparatus for holding the wires. Instead of attaching 

 them to a fixed diaphragm placed between the two lenses of the secondary 

 object-glass, they will be fastened to a sliding diaphragm with two apertures ; 

 across one of the apertures only will be fixed the wires, so that a photograph 

 may be taken either with or without them. No appreciable distortion in the 

 photographic image of the wires can be detected. 



The wires will be two in number; they will cross each other at an angle 

 of 90°, and form an angle of 45° with a circle of declination. This system 

 of wires is the same as that proposed by Mr. Carrington and used in his 

 observations of solar spots. It is intended when the apparatus is complete 

 to observe the contacts of the sun's limb with the wires as it passes them in 

 succession each day before commencing a set of photographs, and also 

 immediately after completing them. In order to observe these contacts, the 

 image of the sun and wires will be received on the ground-glass focusing 

 plate, and the times of the several transits noted by viewing the image of the 

 sun and wires through the plate. One photograph will in all cases be taken 

 with the wires, and two or three without the wires, in order to secure all the 

 details possible, as well of the faculse as of the spots. 



Degree of perfection attained. Stereoscopic pictures of the Sun. — By over- 

 * Report Brit. Assoc. 1857, p. xxxv. 



