1872.] 



Spectrum of the Great Nebula in Orion. 



381 



apparatus with which the observations in this paper were made. In the 

 former paper, to which I have already referred, I gave some reasons* to 

 show that the ordinary method of comparison, by reflecting light into the 

 spectroscope by means of a small prism placed before one half of the slit, 

 is not satisfactory for very delicate observations unless certain precau- 

 tions are taken. I then describe an arrangement for this purpose, which, 

 with one or two modifications, is adopted in the collimator constructed for 

 use with the Royal Society's telescope. I give the description from that 

 paperf : — 



" 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 requirements 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 direction 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 appearing above and the other below it. As the reflect- 

 ing surfaces are about 0'5 inch from the slit, and the rays from the spark 

 are divergent, the light reflected from the pieces of glass will have en- 

 croached 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 micro- 

 scopic objects were carefully selected, and these were silvered by floating 

 them upon the surface of a silvering 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 inch in 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 adapting-tube, by which 

 the apparatus is attached to the telescope, with a diaphragm with a small 

 central hole, before which a spirit-lamp was 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 considered perfect. The accuracy of adjustment 

 has been confirmed by the exact coincidence of the three lines of mag- 

 nesium with the component lines of b in the spectrum of the moon." 



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

 * Phil. Trans. 18G8, pp. 537, 538. t Bid. 1868, p. 538. 



2 f 2 



