94 METHODS OF OBSERVING ABSORPTION SPECTRA. [BOOK I 



marks for future observations. A very useful method of recording the 

 position of lines and bands in the spectrum, lately suggested by Dr 

 MacMunn 1 , is thus described in his own words : 



"The slit of the spectroscope being illuminated by some light, it 

 is sufficiently narrowed, and the eye-piece focussed, till the Frauenhofer lines 

 are seen distinctly; a camera-lucida is then slipped over the eye-piece, and a 

 point marked on a piece of paper placed beneath the camera just beyond 

 the extreme red, and another beyond the extreme violet. A number of 

 blank spectrum maps are then made of this length, and again brought beneath 

 the camera ; the position of the Frauenhofer lines is marked on the top one, 

 and afterwards on all the others. In this way a number of solar 

 maps are made, from which any required number can afterwards be 

 copied. 



" When an absorption spectrum has to be mapped, a test-tube containing 

 the solution, illuminated by means of a strong light, is placed before 

 the slit, the right-angled reflecting prism is made to cover half the slit, and 

 a Bunsen burner, with a salt of sodium introduced into its flame, is placed 

 so that its light shall fall upon the right-angled prism. On looking 

 into the instrument two spectra are seen, one the absorption spectrum, 

 the other the spectrum of sodium a yellow line on a dark background. 

 The camera lucida is then slipped over the eye-piece, two maps with 

 the Frauenhofer lines marked on them brought beneath it, and the paper 

 shifted till the bright-yellow sodium line covers the D line on the 

 maps ; with a lead pencil the position of the bands and the amount of 

 shading is marked on the maps, care being taken to keep the paper 

 from slipping. It must be remembered that the maps have to be 

 turned upside down while being made, otherwise the A line would be on the 

 right-hand side and the H on the left in the solar maps, and the 

 absorption bands in the wrong place in the others." 



Spectro- For some time past physicists have been in the habit 



scopes with o f recording the position of bright or dark lines observed in 

 the spectrum by stating the wave-length of the region 

 in which they occur. Usually the observations have been 

 made with instruments furnished with an arbitrary scale 

 only. Having determined the position of certain lines on the solar 

 spectrum (of which the wave-length is precisely known) in reference to 

 the arbitrary scale, data are obtained for constructing, by an easy geo- 

 metrical process, a curve which represents the relation of any point on the 

 arbitrary scale to a scale of wave-lengths. The observations which have 

 been made with the arbitrary scale are . then reduced to wave-lengths. 

 Though the reduction is somewhat troublesome the observer knows that, 

 when made, his observations have acquired a definiteness which they other- 

 wise would never have possessed. Usually wave-lengths are now express- 

 ed in 10-millionths of a millimetre, but other units of measurement may 

 be employed. 



Recently Herr Carl Zeiss, the eminent optician of Jena, has, at the 

 suggestion of Professor Abbe, constructed spectroscopes provided with an 

 illuminated scale, which is divided and numbered so as to permit of the 



1 MacMunn, Studies in Medical Spectroscopy. Keprinted from the Dublin Journal 

 of Med. Sc., June, 1877. 



