L 384 ] 



XXXI V. The Echelon Spectroscope; with Application to 

 Investigate the Behaviour of the chief Lines in the Mercury 

 Spectrum under the Influence of a Magnetic Field. By 

 Lord Blythswood and Dr. E. W. Marchant, Granville 

 Scholar of the University of London *. 



§ 1. Introduction. 



IT has been thought that it might be of interest to publish 

 some account of the mounting adopted in the Blythswood 

 Laboratory for the echelon diffraction-grating (invented some 

 two years ago by Prof. Michelson f), and to describe the 

 method adopted for the determination of what may be called 

 the constants of the instrument. 



The echelon grating in use here was by Hilger and was 

 the first made in this country. It consists of fifteen plates 

 each about 7*5 mm. thick, made of light flint-glass, set 

 originally with 1 mm. steps, but afterwards altered so as to 

 make the steps only *5 mm. wide. 



§ 2. Method of Mounting. 



The mounting was adapted so as to make use of the tele- 

 scope and collimator belonging to a large goniometer already 

 in use in the laboratory. The telescopes in question had 

 object-glasses of 28 inch focus and 2 inch aperture J. 



In order to obtain the necessary preliminary dispersion a 

 small solar spectroscope was employed, having movable dis- 

 persion- and back-reflexion prisms, so that both the telescope 

 and collimator were fixed. This form of instrument is ex- 

 ceedingly convenient when the source of light cannot easily 

 be shifted. 



A diagram and photograph of the arrangement are given in 



%. i. 



The telescopes T x and T 3 are first set so that the slits 

 Sj, S 2 are in the solar foci of their respective object-glasses. 



The instruments are then so set relatively to each other, 

 that the spectrum from the solar spectroscope focusses exactly 

 on the slit S 2 . In order to get any particular line of the 

 spectrum on the slit S 2 , the handle H is turned (thus 



* Communicated by Lord Kelvin. 



f Astrophysical Journal, vol. viii. No. 1, p. 36, June 1898. 



X When the mounting is made complete for the instrument, it is 

 advisable to have short-focus glasses (say 6 in. or 8 in.), with the same 

 aperture, so as to get as much light as possible. 



