SIR W. CROOKES ON THE PHOTOGRAPHIC SPECTRA OF METEORITES. 
413 
the instrument could be quickly set to give a photograph in perfect detail for any 
desired portion of the spectrum. 
The Slit . 
The slit—of necessity one of the most important parts of the instrument—has been 
subject to more alteration and modification than any other portion. On account of 
the close proximity of my chemical laboratory I decided at tbe outset to avoid the 
use of steel for this purpose ; on this point I differed from Prof. Schumann, who was 
a strong advocate of hardened steel jaws, and all his splendid photographs were 
made with spectrographs having steel slits. I obtained good results by using rolled 
metallic cobalt—very hard and non-corrosive—ultimately I hit upon the plan of 
employing quartz instead of metal. * 
I find this substitute answers so perfectly that I cannot imagine anything better. 
At first I had great difficulty in getting the edge true. The sides and angle must 
be ground true and polished. It is difficult to fashion a knife-edge of quartz at an 
angle of 45°, because small splinters occasionally break off. Ultimately I got over 
this difficulty by putting a very narrow bevel on the front of the plate, so making 
the angle of each jaw 90°, thus (fig. l):— 
I 
i 
Fig. 1. 
Owing to refraction no light can get through the part of tbe quartz which is cut 
at an angle. The edge made in this way is absolutely black and opaque even in 
sunlight, and should bear a high microscopic power without showing irregularity. 
To prevent light from coming through the flat part of the quartz plate, the surface, 
all but the extreme edge, is coated with gold by cathode deposition. It is very 
beautiful to see the absolute opacity and trueness of edge of one of these slits when 
examined under a high power. 
Jaws made in this way have now been in continuous use for many years and give 
perfect satisfaction ; they are absolutely unaffected by the atmosphere, can easily be 
cleaned, and with common care do not wear out. 
The width of slit I use is small ; for general work I use 0'008 mm. only, but for 
years this very fine width was a source of annoyance and anxiety, the reason being 
* ‘Chemical News,’ No. 1846, vol. lxxi., p. 175, April 11, 1895. 
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