Spectrum of the Great Nebula in Orion. 135 



arrangement for this purpose, which, with one or two modifications, 

 is adopted in the collimator constructed for use with the Royal So- 

 ciety's telescope. I give the description from that paper* : — 



"The following arrangement for admitting the light from the 

 spark appeared to me to be free from the objections which have been 

 referred to, and to be in all respects adapted to meet the require- 

 ments of the case. In place of the small prism, two pieces of silvered 

 glass were securely fixed before the slit at an angle of 45°. In a di- 

 rection at right angles to that of the slit, an opening of about ^inch. 

 was left between the pieces of glass for the passage of the pencils 

 from the object-glass. By means of this arrangement the spectrum 

 of a star is seen accompanied by two spectra of comparison, one ap- 

 pearing above and the other below it. As the reflecting surfaces are 

 about 0*5 inch from the slit, and the rays from the spark are diver- 

 gent, the light reflected from the pieces of glass will have encroached 

 upon the pencils from the object-glass by the time they reach the slit, 

 and the upper and lower spectra of comparison will appear to overlap 

 to a small extent the spectrum formed by the light from the object- 

 glass. This condition of things is of great assistance to the eye 

 in forming a judgment as to the absolute coincidence or otherwise 

 of lines. For the purpose of avoiding some inconveniences which 

 would arise from glass of the ordinary thickness, pieces of the thin 

 glass used for the covers of microscopic objects were carefully selected ; 

 and these were silvered by floating them upon the surface of a silver- 

 ing solution. In order to ensure that the induction-spark should 

 always preserve the same position relatively to the mirror, a piece of 

 sheet gutta percha was fixed above the silvered glass ; in the plate of 

 gutta percha, at the proper place, a small hole was made of about 

 ■^jy inch diameter. The ebonite clamp containing the electrodes is so 

 fixed as to permit the point of separation of these to be adjusted 

 exactly over the small hole in the gutta percha. The adjustment of 

 the parts of the apparatus was made by closing the end of the adapt- 

 ing-tube, by which the apparatus is attached to the telescope, with a 

 diaphragm with a small central hole, before which a spirit-lamp wa3 

 placed. When the lines from the induction-spark, in the two spectra 

 of comparison, were seen to overlap exactly, for a short distance, the 

 lines of sodium from the light of the lamp, the adjustment was con- 

 sidered perfect. The accuracy of adjustment has been confirmed by 

 the exact coincidence of the three lines of magnesium with the com- 

 ponent lines of b in the spectrum of the moon." 



The modifications of this plan consist in the substitution of a thin 

 silver plate polished on both surfaces for the pieces of silvered glass. 

 The opposite side of the silver plate to that from which the terres- 

 trial light is reflected to the slit reflects the images formed by the 

 object-glass to the side of the tube where a suitable eyepiece is fixed. 

 This arrangement forms a very convenient finder; for it is easy to 

 cause the image of the star to disappear in the hole in the silver plate. 

 When this is the case, the line of light formed by the star falls on 

 the slit, and its spectrum is visible in the spectroscope. This colli- 

 * Phil. Trans. 1868, p. 538. 



