1872.] ats Imperfections and theiy Remedy. 57 
quartz, cut parallel to the axis of the crystal, and the lines 
could be covered with a similar plate to prevent access of 
dust. By using quartz the extreme ultra violet lines could 
be obtained without absorption, and be rendered visible by 
means of a fluorescent screen. To prevent the effects of 
diffraction between the lines, in taking the photographs, the 
wire frame should have fine, thin, white paper placed behind 
it, through which the light should be admitted to pass 
through the frame, and all other light should be excluded 
from the camera. 
Having thus procured the system of fine equidistant lines, 
the next point is to make such arrangements that a highly 
magnified spectrum could be obtained from them by means 
of a telescope placed at a moderate distance. Suppose a 
box to be formed about 18 inches in length, and that the 
system of fine lines, with its slit and collimator furnished 
with quartz lenses, are placed at the top of the box, about 
the centre, so that the spectrum formed may be thrown 
down to the centre of the interior of the box, an arrangement 
being provided for shifting the system of fine lines along a 
graduated scale, so as to bring successively the different 
parts of the spectrum into view. Suppose a very small 
plane reflector, of silver or speculum metal, to be placed in 
the centre of the interior of the box, at an angle of 45° to- 
wards the roof, to receive the image of the spectrum, and 
let the image formed by this mirror be in the focus of a 
parabolic reflector placed at one end of the box, so thatx., 
the rays reflected from the small plane mirror shall be re- 
flected parallel towards the opposite end of the box. Exact, 
opposite to this parabolic reflector let there be placed a 
reflecting telescope, the parabolic reflector of which shall 
receive the parallel rays, and concentrate them in its own 
focus on another small plane mirror, where they may be 
viewed by the eye-piece to: be composed of quartz lenses. 
By this arrangement the light would never pass through 
any other medium than quartz, which does not absorb the 
ultra-violet rays. The rendering the rays parallel at once 
would obviate the necessity for viewing the spectrum from a 
distance, in order to obtain sufficient magnification, which 
might be secured of any desired amount, by means of the 
reflecting telescope, without much loss of light. For viewing 
the ultra-violet rays a fluorescent surface might be substi- 
tuted for the first of the small plane mirrors. 
The chief practical difficulty would be the procuring of 
two good parabolic reflectors for the instrument ; but it is 
VOL. II. (N.S.) I 
