and Prismatic Spectra on a Screen. 23 
The grating generally used was made by Mr. L. M. Ruther- 
furd; it is ruled on speculum metal, 6,481 lines to the inch ; it 
gives spectra by reflection. Other gratings on glass, now in my 
possession, give spectra by reflection and by transmission. 
The method answers equally well for both. It may be briefly 
stated as follows: 
A beam of light is directed by the silvered plane mirror of 
a heliostat (A) into a darkened room 
t is received on an achromatic lens (B) ten centimeters in 
diameter ; focal distance from posterior surface seventy centi- 
meters. 
A slit (C) is then placed within the focus of this lens, the dis- 
tance being forty-eight centimeters from the lens (B). 
After passing through the narrow slit, which is about one- 
tenth of a millimeter wide, the light is received upon a second 
achromatic lens (D), of the same diameter as the first, but with 
Rs w | 9 7 
E te Cc B A 
a focal distance of one hundred and fifteen centimeters. The 
distance of this lens from the slit is one hundred and sixty-four 
centimeters, and the focussing of the lines of the spectrum on a 
paper screen or on the ground glass of the camera is accom- 
plished by moving the lens (D) nearer to or farther from the 
slit (C), or by moving the camera or screen (F) itself. 
_The grating (E), mounted on a suitable stand, is placed at a 
distance of eighty centimeters from the second lens. All parts 
of the apparatus being carefully adjusted, so that A, B, C, D, E 
are on the same horizontal axis, the grating is then arranged on 
its vertical axis, to throw the center of its reflected image on the 
opening of the slit (C). 
The lines of the grating being accurately parallel to the sides 
of the slit, a series of beautiful spectra are produced on each 
side of the slit, any or all of which may be received on suita- 
bly adjusted screens, one of which is represented at (F). In all 
of these spectra, if the slit has been very narrow, the promi- 
Sa, < raunhofer, with numerous other lines, appear sharply 
efined. 
Of the spectra described above, only the first, second and 
third orders on each side of the image of the slit are available 
for general use on account of the overlapping of those that fol- 
low. Of those that are available, I have preferred to use the 
second order, since in this the dispersion is much greater than 
