156 



NATURE 



[June 17, 1897 



grant the use of a man-of-war to allow an attempt to be 

 made to repeat the Volage programme of 1896. 



In this case the station will possibly be the old fort at 

 Viziadurg, for which point the following astronomical 

 conditions hold, according to the Superintendent of the 

 Nautical Almanac (Fig. 2). 



Assuming the position of Viziadurg to be 16^ 32' N., 

 and 73° 22' E., the times of contact are (local mean 

 time) : — 



d. h. m. s. 

 1898. — January 21 23 12 20 



,, 22 o 46 9 P, = 241° 

 „ 22 o 48 14 P3= 51° 

 ,, 22 2 14 33 

 These times are 4h. 53m. 28s. in advance of Greenwich. 

 The land parties — ^which will include the Astronomer 

 Royal, Prof. Turner and Mr. Newall, representing the 

 Observatories of Greenwich, Oxford and Cambridge, to- 

 gether with Dr. Common and Captain Hills— will occupy 

 stations near the central line on the railways shown on 

 the map (Fig. i). 



With regard to the work to be attempted by the coast 

 party, to which I propose to confine myself in what 

 follows, I may state, in the first place, that I am one of 



Fig. 2.— The conditions of observation at Viziadurg. 



those who believe that spectroscopic observations during 

 eclipses must take precedence of all others in the minds 

 of students of solar physics ; but when I say this, it 

 must not be forgotten that other inquiries remain which 

 are much more simply carried out, and are within the 

 competence of those unacquainted with the details of the 

 subject ; one of the fortunate things about eclipses is 

 that photographers and amateurs can do good work as 

 well as those more fully equipped. 



But to return to the spectroscope — Whatlfoi;m of spectro- 

 scope are we to employ ? 



The Prismatic Cajitera. 

 Fraunhofer, at the beginning of the century, found that 

 in order to observe the spectra of stars the best thing to 

 do was to put a prism outside a telescope, and to let the 

 light enter the telescope and be brought to a focus after 

 it had passed through the prism ; and it is a most un- 

 fortunate thing, that the neglect of the application of this 

 principle has landed us probably in a delay of fifteen or 

 twenty years in gathering knowledge on this subject. 

 Now the spectroscopes with which most are familiar are 

 armed with a slit through which the light to be examined 

 is made to enter, and the rays are next rendered parallel 

 before they enter the prism in a part of the instrument 



NO. 1442, VOL. 56] 



called the collimator. After passing the prism they are 

 again collected to a focus by means of a telescope. 



But a spectroscope need not be so complicated as this, 

 for after all the object of the instrument is to disperse 

 white light as we see it dispersed in a rainbow, and what 

 nature accomplishes by a rain-drop we can do by a prism ; 

 hence, if we simply pass a ray of white light through a 

 prism, we find that after it has so passed through, it is 

 changed into a beautiful band, showing all the colours of 

 the rainbow. This prism then is the fundamental part 

 of the instrument, and the most complicated spectroscope 

 which we can imagine simply utilises the part which the 

 prism plays in breaking up a beam of white light into its 

 constituent parts from the red to the violet. Between 

 these colours we get that string of orange yellow, green, 

 and blue, which we are familiar with in the rainbow. 

 For sixpence any of us may make for ourselves an 

 instrument which will serve the purpose of demon- 

 strating many important spectroscopic results. From 

 an optician we can get a small glass prism for six- 

 pence ; glue it at one end of a piece of wood about 

 12 X I X I inch, so that we can see through it a coloured 

 image of a needle stuck in at the other end of the piece 



Fif;. 3. — A simple form ot spectroscope. 



of wood (Fig. 3). This we must do by looking sideways 

 through it. 



Allow the needle to be illuminated by the flame of a 

 spirit lamp into which salt is gradually allowed to fall. 

 We see an image of the needle coloured in orange. 

 If we next illuminate the needle by a candle or gas 

 flame, taking care that the direct light from the candle 

 does not fall upon the face of the prism, we then get no 

 longer a single image of the needle, but a complete 

 band of colour from red to blue. We have, in fact, an 

 innumerable multitude of images of the needle close 

 together. 



It will be clear from these experiments that in our 

 impromptu spectroscope we see simply images of the 

 needle, few or many, according as the kind of light we 

 are studying contains few or many differently coloured 

 rays. 



In the more complicated instrument we pass from an 

 illuminated needle to a fine straight slit through which 

 light is allowed to enter. We generally talk of " line " 

 spectra for the reason that a narrow slit is employed, the 

 image of which is a line. In the "lines" seen in the 

 spectra of the heavenly bodies we have so many celestial 

 hieroglyphics which we have to translate into chemical 



