NA TURE 



[May 4, 1905 



placed in a separate house the upper portion of which 

 can be rolled back towards the north. Some distance 

 due south of this, in another building, is the 12-inch 

 Taylor photo-visual lens mounted on a concrete pillar, 

 and still further south, and in the same building, is 

 the spectroheliograph, also mounted on concrete 

 pillars. 



Wtih this arrangement the solar beam is thrown 

 by the siderostat mirror continuously due south and 

 in a horizontal direction ; this beam then falls on the 

 12-inch lens, and the solar image in the focus of this 

 lens is thrown on the primary slit plate of the spectro- 

 heliograph. 



In order to analyse the solar image by allowing 

 each portion of it to fall successively on the primary 

 slit, the latter, and consequently the whole of the 

 spectroheliograph, has to be moved horizontally in an 

 east and west direction, a distance a little more than 

 the diameter of the solar image (in this case 

 2I inches). Further, this motion has to be extremely 

 uniform. 



The method adopted to accomplish both of these 

 requirements is as follows : — .\ triangular iron frame- 

 work (Fig. i) is supported on three levelling screws 

 on three concrete pillars. A second framework of the 

 same size and material is placed on the first, but. 

 separated by steel balls free to roll between small 

 steel plates fixed to each framework near the corners. 



The longer side of this isosceles triangle is placed 

 in a north and south direction. The direction of 

 motion of the upper framework is restricted to an 

 east and west line by means of a guide bar fixed to 

 the lower framework; two small levers with rollers 

 attached to the upper framework are pressed against 

 this guide bar by means of small weights, thus en- 

 suring the correct direction. 



The actual motion of the upper framework is 

 obtained by weights attached to one end of a steel 

 strap the other end of which, after passing over a 

 pulley mounted on an arm on the lower framework, 

 is fixed to the western corner of the upper framework. 

 This weight always tends to pull the upper frame- 

 work towards the west, that is towards the right in 

 Fie I. 



The motion is controlled bv a plunger proiecting 

 downwards from the upper framework operating a 

 piston in a cvHnder full of oil attached to the lower 

 framework. The outlet valve can be so adjusted that 

 anv desired rate of motion can be obtained. 



Owing to chang-es of temperature of the oil. 

 different rates of movement can be obtained for any 

 one reading of the micrometer head regulating the 

 outlet valve. It is necessary, therefore, when makinsr 

 an exposure for a " disc " or " limb " picture to take 

 the temperature of the oil into account. This is 

 accomplished bv emploving a table, made from 

 previous "runs," in which the valve setting can be 

 directly read off from the temperature reading and the 

 reauired lensrth of exposure. 



It is on the upner framework that the optical parts 

 of the spectroheliograph are placed. These consist 

 of a double tube carrying- the two slits CFig. 21 at the 

 northern or siderostat end and the two lenses ('4-inrh) 

 of equal focal lenp-fh at the southern end. The dis- 

 persion is produced bv a sinerle prism of 60°, and a 

 reflector is inserted in the system in order to make 

 the total deviation of the beam iSo°. Thus the nart 

 of the solar image which passes through the primarv 

 slit falls on the collimatine lens, is reflected bv the 

 6-inch mirror on to the prism, traverses the latter, and 

 finally, after passing- through the camera lens, is 

 brought to a focus in the plane of the secondarv slit 

 in the form of a spectrum. Bv isolating" anv par- 

 ticular line in this spectrum bv means of the secondarv 

 NO. 1853. VOL. 72] 



slit (Fig. 2) the solar image can be analysed in this 

 wave-length. 



For photographing the -whole disc of the sun or 

 its immediate surroundings with one exposure the 

 lengths of the slits must be greater than the diameter 

 of the solar image (2I inches); in the present case they 

 are 3 inches long. Further, owing to the fact that 

 the lines in the spectrum are curved, the secondarv 

 slit jaws are curved to the same radius ; this necessi- 

 tates very accurate adjustment of the secondary slit 

 on the line, and means are provided to facilitate such 

 requirements. 



In order to obtain a photographic record of the sun 

 in monochromatic light, a fixed photographic plate is 

 held bv means of a wooden support as close to the 

 secondary slit as possible (Fig. 2). In this way, as 

 the primarv slit moves over the stationary solar image, 

 so the secondarv slit traverses with equal speed the 

 sta'tionary photographic plate. 



Up till now the secondary slit has usually been 



Fig. t. — '1 he primary slit is on the left and the secondary behind the plate 

 carrier is sern on the right. This illustration shows also the metal disc 

 in position for a " limb " exposure. 



adjusted on the " K " line of calcium by eye estim- 

 ation aided by a small watchmaker's lens, a check 

 being made by taking a photograph of the spec- 

 trum, if possible with a sun-spot region, on the 

 primary slit. On bright days this setting can be 

 made with little difficulty, but during the late autumn, 

 with a low sun, the " K " region of the spectrum is 

 not easy to see, and the setting is in consequence 

 very uncertain. A new method just brought into 

 operation entirely eliminates this difficulty, for at a 

 constant distance on the red side of the " K " line 

 a small glass plate has been set with a cross engraved 

 on its surface which can be adjusted on a known 

 line in the more visible region of the spectrum. By 

 bisecting a particular line with the cross the " K " 

 line is adjusted on the slit jaw simultaneously. 



The photographs taken during the past year have 

 been of two kinds, the first to investigate the dis- 

 tribution and area of the calcium clouds, or flocculi 

 as Prof. Hale has termed them, on th° sun's disc, and 



