414 
SIR W. CROOKES ON THE PHOTOGRAPHIC SPECTRA OF METEORITES. 
that it was necessary for purposes of adjustment, alignment of the spark, &c., to open 
the slit wide and then close it again. I found that even with the very best screws 
and construction there was always some uncertainty as to the absolute width of the 
slit used in any photograph—thus producing annoying variations in spectra, made at 
different times which should have been identical. During the first ten years of work 
many slits were constructed, each showing a slight improvement; even then the 
uncertainty was not completely removed and the mechanical construction had 
become deeply complicated. Ultimately the difficulties were removed by the device 
that I call the Fixed slit system. This device gives perfect satisfaction and has the 
advantage of extreme simplicity. 
The Fixed Slit System. 
Three such slits were prepared by taking brass blocks all of the same thickness, 
and upon each was permanently fixed a pair of quartz jaws set with openings of 
0'008, 0'025, and 0'06 mm. respectively—these three widths being sufficient to 
embrace all the demands likely to occur. 
The portion of the instrument upon which the slit is fixed is finished in the way 
shown in fig. 2 ; it carries two pointed pins, one of which appears marked C, a steel 
stop E, and a screw ; the whole is closed to light except for a small rectangular 
central opening 1 mm. wide and 10 mm. long. A is one of the brass blocks carrying 
the quartz jaws D ; this block can be instantly attached or removed by a slight turn 
