SIR W. CROOKES ON THE PHOTOGRAPHIC SPECTRA OF METEORITES. 
415 
of the screw, and by the “hole, slot, and plane” device its position is invariable, as 
can be seen. 
In front of the slit and attached to its plate is a circular brass disc B ; this disc 
completely covers the quartz jaws D ; it is pierced with several openings, any one of 
which can, by a touch of the finger, be brought in front of the slit and held there by 
a spring which engages in a notch—-(as shown below). 
By these means light from various sources can be passed into the spectrograph 
through different portions of the slit and ultimately reaches the photographic film. 
For instance, in the particular work that forms the subject of this paper, the disc B 
is so fixed that the aperture marked Fe faces the slit, and the iron arc is projected 
upon it; then when a sufficient exposure has been given the disc is rotated so as to 
bring the aperture marked x opposite the slit, and the light from a silver-aerolite arc 
is passed in, the resulting photograph shows two spectra slightly overlapping, one of 
iron and the other of the silver-aerolite. No disturbance of the spectrograph or of the 
photographic film occurs during the operation. 
A front view of the slit plate A is shown in plan in fig. 3 ; the dotted circle is the 
part covered by the disc. 
The jaws do not wear or corrode. It is only necessary to clean and free them from 
dust by drawing between a sharply cut wedge of peg-wood. # 
Before enumerating the metallic constituents of the aerolites I wish to place on 
* Peg-wood splints are used by watchmakers to clean out pivot-holes in watches. 
