Total Solar Eclipse of 1901, May 17-18. 231 



The instrument employed was in essentials that used at Algiers in 

 1900, but it was mounted differently. 



A plane grating by Rowland, 14,438 lines to the inch on a ruled 

 surface 5 x 3| inches, was fitted on an axis in front of a telescope of 

 aperture 4 inches and focal length 66*8 inches. The grating is a very 

 brilliant one ; the objective is an excellent one, corrected for photo- 

 graphic rays, by Cooke and Sons ; and both belong to the splendid 

 spectroscopic installation arranged by the late Professor Piazzi Smyth. 

 I am under great obligation to the Royal Society for the continued 

 loan of this outfit. 



The telescope with grating attached was fixed inclined at an angle 

 of 20 to the horizontal and pointing downwards on the sloping pier 

 which supported the double camera, the telescope and camera being 

 parallel. Part of the beam reflected by the coelostat was allowed to 

 fall on a 6-inch mirror, and thereby reflected downwards on to the 

 grating. By inclining the plane of this reflection by the proper 

 amount with the vertical, it was possible to get the crescent at second 

 contact symmetrically disposed with respect to the plane of dispersion 

 of the grating. It was thought best to run the risk involved in the 

 loss of light for the sake of getting the telescope into the very con- 

 venient position of parallelism with the double camera, and on the 

 same sloping pier with it. 



The light reflected by the small mirror fell upon the grating with an 

 incidence of 39, and the diffracted beam left the grating at an angle 

 of about 5 on the other side of the normal. In this position H v in the 

 spectrum of the second order was not far from the centre of the field 

 of view of the telescope. 



The focussing and final adjustment of the spectrum on the focussing 

 plate .was intentionally left to be done a few minutes before totality, 

 when the width of the diminishing crescent would make it possible to 

 recognise the exact part of the spectrum in the field of view. Ten 

 minutes before totality the cloud was so obscuring that on looking into 

 the eye-piece I could not distinguish between the spectrum of the 

 crescent and light reflected from the clouds. Returning to the instru- 

 ment 6 minutes later I found the lines visible, and Lieutenant Briggs, 

 who was to take charge of the exposures with this instrument, turned 

 the tilting screw of the grating till I could see that the dark crescent- 

 shaped lines near A 4231 were in the centre of the field. I then 

 focussed very carefully, and handed over the instrument to Lieutenant 

 Briggs. 



Programme. Exposures were to be made 



(i) For 1 second when the signal " Stand by " was called by 

 Dr. Wallace 15 seconds before totality. 



(ii) For 2 seconds when the count " Thirteen " was called before 

 totality. 



