324 
CAPTAIN L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUNDER 
The purity was such, therefore, that near the wave-length, 4 X 10 ^ two lines 
differing by the six-thousandth part of their own wave-length should be resolved. 
Spectroscope II. 
cm. 
Focal lens of condenser to form image of the corona 
on the slit. 26'5 
Focal lens of collimator for G.32'5 
,, ,, telescope ,,.20‘0 
Aggregate thickness of two prisms. lO’O 
Width of slit (a). ‘01 
The beam was limited by the prisms, the aperture of the collimator and camera 
being more than sufficient to fill the prisms and transmit the beam. 
The calculation is carried on as before, except that we must now take the useful 
instead of the full aperture of the collimator in order to find As the prisms were 
placed in minimum deviation for G, this can be calculated, and we find it to be 2‘4 cm.; 
this determines 
, -01 X 2-4 
= —- X - -■ = 74 X 10-t 
o2-o 
P = PX/(X -f axlf) 2K/39 ; 
or, roughly speaking, the purity in this case was only the twentieth part of what it 
might have been if the slit had been narrower. The spectroscope wms intended 
originally for the outer parts of the corona, and the slit was, therefore, intentionally 
kept rather wide. There was at the time no means of measuring the width of the 
slit, and I did not realise how much of the resolving power was lost by widening the 
slit. The full resolving power in the violet was 32, and therefore the purity 
realised P6. 
2. Adjustments of Instr uments. 
In order to be able to differentiate between the spectra of different parts of the 
corona, its image must be thrown on the slit of the spectroscojDe ; and for that purpose 
the plane of the slit must be placed in the focal plane of a condensing lens. This Avas 
done as follows : an auxiliary telescope Avas focussed for parallel rays, and the slit of 
the spectroscope looked at from behind through the condensing lens, which AAms moA^ed 
backAvards and forAvards until a sharp image of the slit Avas seen in the telescope. 
The method is the same as that usually employed for adjusting the collimator of a 
spectroscope; only that, instead of observing the slit through the collimating lens, 
AA^e observe it through the condensing lens. 
