IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 8038 
nished with a wheel of coloured glasses, so as to allow the observer to make the 
reference-line of almost any colour he should please. 
This wheel promised to be a charming addition, characteristic too for 
spectroscopy ; and yet herein was ultimately found the weakness. For as the 
observer advanced from the red, to the violet, end of the spectrum, if there were 
any fine lines to measure there, they had more and more parallax (or the 
bright pointer line on them) ; until at last they seemed to roll from side to side 
with every movement of the eye at the eye-hole of the ocular, in a manner to 
defy all attempts at accuracy; and no change in the colour wheel could 
cure it. | 
After vainly trying decreased eye-holes, grayed surfaces for the wedge bear- 
ing the bright line, and every likely looking bit of blue, violet or lavender glass, 
from French, as well as English, makers, the following three steps were taken,— 
and were practically accomplished for me, I am happy to say by Mr Hincer 
himself, just as enthusiastically as if they had been to promote, rather than 
supplant, his own long approved bright-line arrangement. 
The jirst step was, that to secure having a good black-line pointer for a bright 
field of view, a steel blade was prepared, cut to an angle of 45° at the top, and 
placed edgeways. It formed, with Mr Hitcer’s excellent workmanship in such 
extra-hard material, a finer, surer line than anything scratched through black 
varnish; and, when optical and mechanical adjustments were properly attended 
to, the extreme point of the blade, in the centre of the field of view, could be 
brought into perfect focus of both eye-piece and object-glass. It remained there 
too without any parallax through the whole of the brighter part of the Solar 
spectrum ; or for as long as the pointer itself could still be seen, dark, on the 
darkening ends of the spectrum. 
The second step was, that in order to turn at pleasure that dark, into a bright, 
pointer, the angle of the upper end of the steel, already well polished, was made 
to reflect to the eye the light of a lamp shining into the eye-piece from just 
above it. And it did reflect that light, but with the most absurd amount of 
parallax whenever the blue, violet or lavender regions of any spectrum were 
under observation. Thus if the steel, as a black-pointer, was placed in the 
middle of the violet-hydrogen’s bright line, one could actually see the black 
point remain fixed there ; while its lamp-illumined face, no matter what coloured 
glasses that illumining light was passed through, rolled from side to side. 
The third step was, that in order to correct such a monstrous defect, the 
wheel of coloured glasses (with their chemical tints no doubt exceedingly vivid, 
but of a diferent refrangibility to the same eye-matched tints of the spectrum), 
was removed ; and in its place was established a small spectroscope pivoting on 
the level of its slit, and so “lensed” as to throw an image of that slit, through 
its direct vision prism upon the steel point ; while a side screw altered the angle 
VOL. XXVIII. PART III. 10 A 
