1910 - 11 .] Absorption of Light by Inorganic Salts. No. IV. 549 
This procedure was repeated then for other ratios of the relative distances 
of mantle and ground glass, and thus A was determined for different 
wave-lengths. 
This method was tested for cobalt fluoride by comparison with the 
spectrophotometer for the blue and green regions of the spectrum, and 
although perhaps not quite so accurate as the latter, it proved very 
satisfactory. It might be thought that it would be slow and laborious. 
But this is not the case ; the exposures only took three minutes. Twelve 
could be taken on one plate, and if we were dealing with an absorption band, 
one exposure could give A for two points, one on each side of the band. 
This apparatus seems eminently suited for doing the spectrophotometry of 
the salts of the rare earths, which has as yet not been attempted. The 
bands of the rare earths are so narrow that the ordinary spectrophoto- 
meters cannot very well be applied to them. 
When we were convinced of the suitability of the inverse square method 
we turned our attention towards the necessary quartz spectrograph. This 
we built ourselves, partly in the interests of economy and partly because we 
thought we could improve on the usual type. The diagram shows our first 
spectrograph ; the lenses are replaced by concave mirrors M, M'. The light 
from the slit is made parallel by M, falls on a plane mirror S, passes through 
the prism at minimum deviation, and is focussed by M' on the photographic 
plate P. This spectrograph will be described when it is fully successful, as 
we are convinced of its advantages. But until now we have had trouble 
with the mirrors ; silver does not reflect the ultra-violet well. Considerable 
time was wasted in waiting for nickel mirrors ; when they came, we found 
they were silver mirrors with nickel deposited electrolytically on them and 
then all polished off again. As we could not afford to wait longer, we built 
