360 Prof. II. II. Turner and Mr. H. K X.-wnll. 



at such a distance from it that tin- grating could U> mounted free to 

 turn on a spindle passing through the sides of the yoke at right 

 angles to the collimation axn. A >Tnall brass cup or socket was 

 attached to the middle point of the yoke so that it lay in the axis of 

 collimation, and it was made the lower 1 tea ring, by which the whole 

 instrument was supported on a pointed pivot, fixed, with a small 

 amount of freedom for adjustment, on a low pillar of Im't kwork. The 

 upper end of the tule of the telescope rested on antifriction rollers, 

 supported on the west side of the higher pillar of 1 trick work, which 

 also carried the four-prism spectroscope. Thus the polar axes of the 

 two instruments, viz., the objective-grating camera and the mounting 

 of the four-prism spectroscope, were side l>y side ; and it was not a 

 difficult matter to link together the two mountings by means of a 

 connecting rod, so that the same clockwork should drive both. Each 

 mounting was connected by slow motions with the one clock-driven 

 sector, and so each could be adjusted relatively to the sun without 

 disturbing the other. The arrangement worked admirably. 



The light of the corona was incident on the grating at an angle of 

 about 55, and the diffracted beam utilised in the telescope left the 

 grating at an angle of about 13 40'. In this position of the grating 

 the green of the second order was used and the magnifying power of the 

 grating was a little greater than one-half, so that the coronal ring was 

 distorted into an ellipse, in which the major axis was perpendicular to 

 the length of the spectrum and parallel to the direction of daily 

 motion. 



The axis of the instrument having been adjusted to parallelism 

 with the earth's axis, it remained only (i) to set the grating so that 

 the coronal ring should appeal 1 in the middle of the field, and (ii) to 

 focus the instrument. Neither of these operations could be done 

 satisfactorily before the eclipse, that is, before the diminishing crescent 

 of the sun made it possible to recognise the exact position of the 

 spectrum in the field of view. Ten minutes before totality the dark 

 lines were indistinctly visible in the spectrum, and a glance showed 

 me that I had had an extraordinary stroke of good fortune in the 

 rough setting of the grating, an operation which had been done by 

 turning the grating till I thought the colour of the green was about 

 right for the background of the magnesium lines. For the lines were 

 only slightly displaced from the centre of the field, and the adjust- 

 ment for the part of the spectrum required in the photograph was 

 practically correct to a nicety. Accordingly no further adjustment 

 was attempted. Two minutes before the beginning of totality the 

 crescent was fine enough to show the dark lines in the spectrum very 

 distinctly, a somewhat bewildering array of interlacing elliptical 

 crescents, and the focussing was accomplished with ease. Mr. Henn 

 then took charge of the instrument, and put a dark slide in position, 



