1874.] On, Displacement of the Solar Spectrum. 219 



Bengal, and the south-easterly winds diminishing in force as they reach 

 the North-west Provinces, indicating that descending and ascending cur- 

 rents must be formed in the upper strata, though of the return south- 

 ward of any descending current from the north there is no direct 

 evidence. 



II. " Note on Displacement of the Solar Spectrum." By J. 

 H. N. HENNESSEY,, F.R.A.S. Communicated by Professor 

 G. G. STOKES, Sec. R.S. Received December 15, 1873. 



The following experiments were made with the (new) spectroscope 

 (three prisms) of the Royal Society to ascertain for this instrument the 

 amount of displacement in the solar spectrum from change of tempera- 

 ture. The spectroscope was set up on a pillar within a small tent at a 

 time of the year when the thermal range is considerable : the collimator 

 was placed horizontal, and directed through a window in the tent to a 

 heliostat, which was made to reflect the sun's image when required. On 

 closing the window darkness prevailed in the tent, so that the bright 

 sodium lines were easily obtained from a spirit-lamp. Before commencing, 

 the slit was adjusted and the spectroscope clamped ; and no movement of 

 any kind was permitted in the instrument during the experiments. The 

 displacement was measured by means of a micrometer in the eye-end of 

 the telescope, reading being taken (out of curiosity) successively to both 

 dark and bright lines, i. e. to K 1002-8=D r and K IOQ6'S=~D V . A 

 verified thermometer was suspended directly over and almost touching 

 the prisms. The meteorological observatory referred to was some fifty 

 yards north of the tent. 



Rejecting observation 5 (in the following Table) because the thermo- 

 meter was evidently in advance of the prisms, we deduce 



By Dark lines, displacement equal 



T)r to T>v is produced by .... 31*3 change of temperature. 

 By Bright lines, displacement equal 



Dr to Dv is produced by .... 29'4 



Mean 30 



from which it appears that the displacement in question may not be 

 neglected in investigations made under a considerable thermal range, 



