LYMAN. — THE SPECTRUM OF HYDROGEN. 127 
THE SPECTROSCOPE. 
The apparatus consists of two parts, the spectroscope itself and the vacuum 
receiver in which it is enclosed. The spectroscope is formed of a drawn brass lube 
9.1 cm. in internal diameter, 96 cm. long, and 1.5 mm. thick, one end of which is pro- 
vided with an arrangement for holding the grating while the other end carries the 
plate-holder and slits. The grating mounting consists of a square bra-s plate pivoted 
to turn about a vertical axis. The grating is held against this plate by springs, while 
screws through the back of the plate permit of the necessary adjustment about a 
horizontal axis. At the end not occupied by the grating a draw tube fits into the 
large tube. Upon the end of this draw tube are mounted the slits and plate-holder in 
a manner shown in Plate VI, figs. 1, 2, and 4, and which may be d scribed as follows 
A circular brass disc closes the end of the draw tube and is pivoted about the points 
A A, fig. 4. The motion of this disc is regulated by the screws X X. Upon the 
disc are mounted the two slits S S. The width of the slits is controlled by the usual 
screw adjustment. In order to be able to adjust the slits parallel to each other one 
of them is mounted in a tube which turns in the disc ; the amount of this twist is 
regulated by the lever L. 
The plate-holder C is so constructed that several photographs may be taken with- 
out withdrawing it from the apparatus. To this end the disc carries two ways D L> \n 
which the plate-holder slides. The position of the holder in the ways is controlled b\ 
the lever E, pivoted about the point F. One end of this lever carries the pin G, while 
the other end is provided with an iron armature // The pin G engages one of the 
horizontal rods, i, and thus holds the plate-holder in position. To shift this position 
it is only necessary to swing the lever about F by means of a magnet exterior to the 
apparatus, the pin G then slips past one of the rods, /, and the plate-holder falls by 
an amount corresponding to the distance between two rods. The plate-holder is 
designed to Dermit the nlate to be bent to the arc of a circle of given curvature. 
perm 
To this end it is constructed in two parts, the outside case C and the movable 
form M. The form (shown withdrawn from the case, fig. 3 ) carries two strips N 'N, 
whose under sides are cut to the desired curvature, the ends of these strips project 
beyond the main body of the form. The plate P is slipped into t 
b 
when unbent, to the curved strips at their middle point 
ie form and 
The form 
drawn into the case by means of the screws 0, the ends of the plate come up 
against the shoulders R R, and as the screws are tightened the plate is bent to coin- 
cide with the strips N N. 
