A XEW SPECTROSCOPE AND SPECTR 



\PH. 



By R. A. HOUSTOUX. M.A.. Ph.D.. D.Sc. 

 (Xatural Philosophy Dcfxirtnuiit. University of Glasgoic.) 



Figure 1. 



The tvpe of spectroscope for general use in ph\sical 

 or chemical laboratories is now prettv well fixed. 

 There is a telescope and collimator, both with 

 achromatic glass lenses, the collimator being fi.xed, 

 and the telescope moving round a divided circle. 

 If the spectrum is to be photogra[)hed, the evepiece 



end of the telescope is 

 replaced by a box carry- 

 ing a photographic plate 

 at its end. With an in- 

 strument of this kind 

 work can be done in the 

 visible spectrum and in 

 the ultra-violet to about 

 JjOuu : if we wish to go 

 further, quartz lenses and 

 a quartz prism must be 

 used. Quartz lenses and 

 a quartz prism will also 

 carry us considerably further into the infra-red than 

 glass. But the quartz prism in general use. the 

 Cornu double prism, gives a sharp image onh' w hen 

 set at minimum deviation, 

 and the focal length of a 

 quartz lens varies so rapidlv 

 with the wave-length that 

 the photographic plate must 

 be set with its surface at an 

 angle of about 21° to the 

 axis of the camera. The 

 focussing of the plate may 

 thus be a lengthv process. 

 On account of these compli- 

 cations and the cost of the 

 special apparatus involved 

 moderate means and limited experience 

 avoids the ultra-violet and infra-red. 



The object of this short paper is to describe a 

 cheap and simple form of spectroscope of radically 

 different design, which is eminenth" suited for the 

 amateur who wishes to work in these regions, and 

 which is also suitable for the visible spectrum. It 

 does not appear to be known in this countrv, although 

 frequently used in research work in America. In 

 this instrument the lenses are replaced bv mirrors, 

 and the Wadsworth mirror-prism combination is 

 used. The Wadsworth mirror-prism combination 

 consists of a prism and mirror mounted together on 

 the prism table, with the plane of the mirror and 

 the plane that bisects the refracting angle of the 

 prism both meeting in the axis of rotation of the 

 prism table. The diagram (see Figure 1 1 illustrates 

 a special case of the arrangement : ED and CA are 



the experimenter of 

 usualh" 



respectively the traces of the two planes referred to, 

 and they both meet in A. the point through which 

 the axis of rotation of the table passes. 



Now consider any ray FGHJK (see Figure 2) 

 passing through the prism at minimum deviation, 

 and being reflected by the mirror. Its path through 

 the prism. GH, is parallel to the base of the prism, 

 DB. and JK is parallel to FG. From A draw AP, 

 AN and AM perpendicular respectively to FG. HJ 

 and JM. Then by symmetr\- AP = .\X. and by 

 equal triangles AM = AX. Consequentlx' .\P = AM. 

 Suppose that the ray FP is white light : the colour 

 in this rav that sufters minimum deviation emerges 

 along JK after passing through the system. Rotate 

 the prism and mirror through the same small angle, 

 keeping the direction FP fixed. Then AP and AM 

 are fixed, and consequently JK is fixed. A different 

 colour now suffers minimum deviation, but emerges 

 along the same straight line JK. 



Suppose now that the single ray FP is replaced 

 b\- a beam of parallel rays and that the prism table 

 is rotated : each colour in turn, as it suffers minimum 



deviation, is unde\'iated and 

 at the same time suffers 

 the same constant parallel 

 displacement. 



The next diagram (see 

 Figure 3) shows how these 

 properties are taken advan- 

 tage of. .A BCD is a solidly- 

 made box, the lid of which 

 has been removed and into 

 which we are looking 

 verticalh' down. S is a 

 slit attached to a piece of brass tubing which slides 

 in a short piece of tube fixed in the side of the 

 box. The light from the slit is rendered parallel by 



Figure 3. 



87 



