142 



KNOWLEDGE. 



Apri 



1913. 



logical Institute, when, in 1904, he accepted the 

 Treasurership and by untiring effort succeeded in 

 placing the finances on a sound basis. He also 

 acted as Treasurer of the Universal Races Congress 



of 1911, and at the time of his death was serving as 

 Assistant Treasurer of the Congress of Americanists. 

 In 1909 he was elected a foreign associate of the 

 Anthropological Society of Paris. 



A NEW GRATING SPECTROGRAPH. 



By A. H. STUART, B.Sc, F.R.A.S. 



Those of us who have read how Fraunhofer made diffraction 

 gratings by winding silver wire of diameter -041111X1. on brass 

 frames and then, with them, measured the wavelengths of lines 

 in the solar spectrum with a surprising degree of accuracy, have 

 envied little except his patience. We have only envied others 

 their apparatus when we have read of the wonderful reflec- 

 tion gratings made by Rowland, and the huge map of the 

 sun's spectrum which he obtained by their aid. This map set 

 the pace, so to speak, in this class of work, and is still of con- 

 siderable value as a standard, in spite of the classical work 

 which Michelson has done with his interferometer. It is the 

 dealers' catalogues that are responsible for stifling our 

 enthusiasm for work with reflection gratings, for even a small 

 instrument of this type costs from £\0 to £20. By the 

 judicious expenditure of £l, however, I have been able to 

 construct a spectrograph on this principle. 



Figure 140 shews this instrument in diagram. S is the slit of 

 the instrument, L is an achromatic lens (2-ins. diameter 

 and 30-in. focal length, value 3s. 6d). Immediately behind 

 L is placed a transmission grating (a moulded replica on 



glass, value 10s. 6d., 

 gives good results), 

 with the prepared 

 surface as close to 

 the lens as possible. 

 Behind this again, is 

 placed a plane mirror 

 M. C is a camera- 

 back upon which 

 the spectrum produced 

 is focused. The dis- 

 tance from L to S and 

 L to C should be 

 equal to the focal 

 length of the lens L. 

 The whole is con- 

 tained in a light-tight 

 wooden box, PORS 

 in figure. 



Now in order to get 

 the various parts of the 

 instrument correctly 

 fixed, it is necessary 

 to consider in detail 

 what happens to a 

 pencil of light when it 

 falls on to the grating. 

 For our purpose we 

 may neglect all except 

 the spectra of the first 

 order, and it will be 

 convenient to consider 

 the rays which have 

 suffered the least 

 deviation (with a grat- 

 ing the violet end of 

 the spectrum is devi- 

 lens all rays in the 

 Now in 



"W 



Figure 140. 



9' 



;b 



>» 



ated the least). If we use a glass 



ultra-violet beyond \ = 3600 will be absorbed. 



Figure 141 the light travels down from S and falls on the 



grating G normally. A large portion of this light passes 



through the grating unchanged, and falls on the mirror M at 



A. If it meet the mirror normally it will be reflected back to 



the grating and a spectrum will pass out towards C. This is 



the spectrum which is to reach the camera. Other spectra 



are, however, formed, and these must be avoided. Those 



which are deviated to the right 



may be neglected, since they ci 



are absorbed by the side of the 



containing box (which should 



be blackened). When the 



light first falls on the grating 



a spectrum will be deviated to 



the left and will be reflected by 



the mirror towards B. The 



mirror must be so placed that 



this spectrum does not fall on 



the lens. Using a grating 



having 14,438 lines to the inch, 



the angle 8 for X = 3600 is 



about 14° 39', and in order 



that the ray B may just miss 



a lens 2 inches in diameter, 



the mirror must be placed 



3-9 inches behind the lens. 



There is yet another spectrum 



to be considered. The light 



falling on the grating in the 



first case will form a faint 



reflection spectrum which will, 



under the present conditions, 



be more or less superimposed 



on the spectrum we want to 



photograph. To avoid this I 



have found it convenient to 



retain the grating at right 



angles to the incident 



light, but to have the mirror slightly twisted, as shewn 



in Figure 140. This will separate the spectrum we want 



from the faint one caused by reflection from the grating 



surface. 



We thus have in the camera a pure spectrum of considerable 

 dispersion at a very trifling instrumental cost and the sacrifice 

 of a little light. 



It is wise to hang a black screen across the box, so that the 

 lens just protrudes; this will absorb any stray reflections from 

 the mirror. 



By having a piece of wood just large enough to fit into the 

 rebates of the camera back, and mounting in the middle of it 

 an ordinary telescope eyepiece, the instrument may be used 

 as a spectroscope. I have found it convenient to mount the 

 mirror (M in Figure 140) on a turntable, and to have a long 

 lever attachment (very much geared down) regulated from 

 near the eyepiece. By this means a steady movement of the 

 mirror will cause the whole of the visible spectrum to move 

 slowly across the field of view. 



'i»; 



A 



m 



Figure 141. 



