Page:Proceedings of the Royal Society of London Vol 69.djvu/247

Rh 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.