450 
Proceedings of the Royal Society of Edinburgh. [Sess. 
Grating spectroscopes, on account of the simple linear relation which 
exists between their dispersion and wave-length, are almost exclusively 
employed for this work. Owing, however, to the overlapping which occurs 
with the spectra of different orders, special precautions have to be taken 
to prevent lines being ascribed to a wrong part of the spectrum. The 
ordinary dry plate is fairly sensitive between the limits A = 2100 and 
A = 5200. Even beyond A = 2100 bright lines may be photographed, 
although the absorption due to the gelatine film somewhat rapidly reduces 
the sensitiveness of the plate. The range of action is equally ill-defined 
Order. 
Wave Length in First Order Spectrum ■— 
( | | | | 
2000 4000 6000 8000 10000 11000 
1 1 1 1 1 
1. 
2. 
3. 
4. 
5. 
6 . 
Fig. 2. — -Overlapping Spectra. 
on the less refrangible side. Ordinary plates will photograph the green 
and yellow regions of the spectrum if given sufficient exposure. Accord- 
ingly, the appearance of a spectrum line on an ordinary photographic plate 
is no evidence whatever that the wave-length lies between 2100 and 5200. 
Fig. 2 shows the overlapping of spectra in the working range of the 
Gottingen instrument. The shaded portion of each spectrum shows the 
region within which photographs of lines may be obtained on ordinary 
plates with reasonable exposures. A deeper shading indicates the parts 
where the light is of comparatively high photographic activity. From the 
figure it will be seen that if one desired to photograph the pure second 
order spectrum from A = 2000 to A = 6000, precautions would require to be 
taken to cut out the first order spectrum from A = 4000 to A = 6000, the 
