072 
DR,. W. HUGGINS ON THE PHOTOGRAPHIC SPECTRA OF STARS. 
of the principal focus of the large speculum. The grooves in which the apparatus 
slides are graduated, so that the apparatus after removal can be replaced exactly in its 
former position. A final determination of this position was made from the definition 
of photographs of star spectra. 
For the adjustment of the collimator of the spectrum apparatus in the optical axis 
of the large speculum a tube six inches long with cross wires at both ends was fixed 
on the collimator and parallel to it. Advantage was then taken of the hole in the 
large speculum. The eye-tube of the Cassegrain was removed and a small Galilean 
telescope, magnifying 16 diameters, was fixed in its place. The spectroscope was then 
so adjusted by suitable screws that the cross wires at the first end of the small tube 
were made to coincide, when viewed through the Galilean telescope, with those at 
the farther end of the tube. 
§ III. Methods of Work 
Two requirements at once presented themselves of such primary importance that 
success was seen to depend upon the perfection of the method adopted to meet them. 
It was necessary to have a method of bringing the image of star readily and with 
exactness upon any desired part of the slit. Further, it was necessary to have a 
convenient method of continuously watching the star’s image when upon the slit 
during the whole time of the photographic exposure, in order to correct by hand any 
small inequalities of the motion of the telescope which might throw the star’s image 
off the slit. In a large equatorial there are other sources of small inequalities of 
motion besides those due to want of uniformity in the clock itself. When it is 
remembered that the star’s image must remain steadily upon a slit of -g^-gth an inch 
in width for perhaps an hour, it will be seen how necessary is the power of continuous 
supervision and of instant control by hand. The following methods were perfectly 
successful. 
A round thin plate of polished silver (l, fig. 1; c, fig. 2), 1|- inch in diameter, with 
a narrow opening in the middle rather longer and wider than the slit itself, was fixed 
over the slit of the spectroscope. This plate forms a plane mirror, and when the 
telescope has been brought approximately into position by its finders, the bright image 
of the star is seen somewhere upon the plate by looking into the small Galilean 
telescope fixed in the place of the eye-piece of the Cassegrain telescope. Now if at 
the same time artificial light is thrown upon the plate, it becomes itself visible, and 
then the opening in it and the slit within the opening can be distinctly seen at the 
same time as the image of the star as a bright point upon it. By the aid of this 
arrangement there is no difficulty in bringing the star’s image by the slow motion 
handles of the equatorial readily and with precision upon any part of the slit. 
As the position of the star’s image even upon the slit itself can be seen, the image 
being somewhat wider than the slit and therefore not wholly lost within it, it is 
