ig: 



KNOWLEDGE & SCIENTIFIC NEWS. 



[A I- GUST, 1905. 



by means of a siderostat. In fact, the complete instru- 

 ment consists of a siderostat to constantly throw the 

 solar rays horizontally in a due south direction, a lens 

 to form the image of the sun, and the spectroheliograph 

 to obtain monochromatic pictures of this image. 



The siderostat (Fig. 2) has a mirror of 18 inches 

 diameter, and the lens an aperture of 12 inches, with a 

 focal length of 18 feet. The solar image thus formed 

 has a diameter of 2 i-7th inches, which Is the same size 

 as that of the monochromatic image photographed. 



To secure the latter the optical arrangement is as 

 follows: — ^^The stationary solar image falls on a slit 

 plate with jaws, 3 inches lonjj, mounted at the no.'th end 

 of a tube, while at the other end of this tube is a lens 

 4 inches aperture and 6 feet focal length; this forms the 

 collimater. The light, after traversing this collimater, 

 then impinges on a plane vertical mirror and is reflected 

 on to a prism. This prism is so placed that the light, 

 after passing through it, falls on to another 4-mch 

 object glass of 6 feet focal length, mounted at one end 

 of another tube similar and parallel to the collimater. 

 In the focal plane of this objective, in which a spectrum 

 is formed, a second slit with jaws 3^ inches long is 

 placed in position. By so adjusting this secondary slit 

 any particular line in the spectrum can be made to pass 

 through the jaws by itself. In this way a line in the 

 spectrum of calcium, or hydrogen, or iron, &c., can 

 be isolated. The lines of the spectrum formed in the 

 above manner are not straight but curved, so that it 

 becomes necessary to employ a slit, the jaws of which 

 are curved to the same amount. .Such a slit rcf|uires, 



Flu. J. (icncral View of the Ijirgc SldtroiUt .,howln)t the i8-lnch 

 Plane Mirror. The upper porilon of the Mouse Is here moved on 

 iU ralU towards the north. 



in consequence, very careful adjustment, and the means 

 adopted for placing the jaws in any required position 

 can be well seen in the accompanying figure (Fig. 3). 



By means, then, of the above optical arrangement, 

 any particular strip of the sun's image which passes 



through the jaws of the first or primary slit issues 

 through the secondary slit as light of one wave-length 

 or colour. 



If the whole spectroheliograph be graduallv moved 

 across the solar image different strips would enter the 



Fig. 3.— The "Secondary" 51it, showing the various Screw-adiust- 

 .'ments available for setting thecurvedj&wscxactly on the<" K." line. 



primary slit, and they would build up a picture of the 

 sun in one wave-length at the second slit. 



In order to produce this change of position in relation 

 to the fixed solar image, the slits and optical parts arc 

 mounted bodiiy on a movable platform. This platform 

 (see Plate i, Fig. i) rests on three balls, each of which 

 is capable of movement between steel surfaces, the 

 lower ones being fixed to the upper surface of another 

 triangular framework supported by three concrete 

 columns. To ensure timform motion — a very im- 

 portant consideration — the movement, which is pro- 

 duced by falling weights, is controlled by the flow of 

 oil through an aperture, the size of which can be varied 

 at will. The direction of the motion required, namely, 

 that in a horizontal direction and at right angles to the 

 axis of the solar beam falling on the primary slit, is 

 obtained by pressure of the upper platform against a 

 guide bar fixed on the lower framework in the correct 

 direction. The photographic plate, like the sol.ir image 

 on the primary slit, must be fixed relatively to the 

 spectroheliograph. This is accomplished by placing 

 firmly on the concrete column a vertical mahogany slide 

 into which the plate holder can be placed as close up to 

 the secondary slit as possible without actually touching 

 any portion of it. 



The method of procedure adopted to secure a disc 

 picture with this apparatus is as follows : — 



The adjustment of the secondary slit to isolate the 

 centre of the " K " line being made, this slit is closed 

 to the required width. The primary slit is next placed 

 in the meridian and the solar image brought by the 

 slow motions central on the slit. This image is then 

 carefully adjusted for focus. The shutter holiirul the 

 primary slit is then closed. 



