ON THE TOTAL SOLAR ECLIPSE OF AUGUST 29, 1886. 
323 
the slit will not diminish the intensity until the width of the slit, as calculated from 
geometrical optics, is equal to the width of a diffraction band. In that case ai// = X 
and the purity is half the resolving power. Hence we shall, when light is of con¬ 
sideration, only be able to make use of half the resolving power of a spectroscope. 
Whether slits which are wider than the limit here given show the fainter lines on a 
photograph more distinctly owing to their increased width is a matter for investiga¬ 
tion ; but it would, perhaps, be well in a future eclipse to adopt that width of slit 
which makes axjj = X. 
Two spectroscopes were to be used on the present occasion, and the following 
numerical data will allow us to calculate their resolving power and the purity of the 
spectrum which they gave :— 
S2Jectroscoj)e I. 
cm. 
Focal lens of condenser to form image of the corona 
on the slit. 34'0 
Focal lens of collimator.lOG'O 
Aperture of „ . 6'0 
Width of slit (a) . ’003 
Greatest thickness of prism. 13’2 
Focal leno-th of camera lens.83’1 
The beam was limited by the aperture of the collimator lens. As the full aperture 
of the collimator was used, we must put t// = 6/106 and axfj 17 X 10“^ ; hence, for 
a wave-length of 4 X 10“^ 
X/(X + axp) = 4/21. 
With the width of slit used, we have, therefore, made use of the fifth part of the 
resolving power. To calculate the latter, we have, in the first place, to consider that 
the effective thickness of the prism used was only 11’4 cm., as the aperture of the 
collimator lens was not sufficient to use the full width of the prism. The dispersive 
power of the glass was sufficiently near to that for which Lord E-ayleigh has 
calculated the value 1’02 cm. as the necessary thickness to resolve the sodium lines. 
As, according to our definition, a resolving power of ’98 is required to separate the 
sodium lines, we may take the prism to give a resolving power of 1 for each 
centimeter thickness in the neighbourhood of the sodium lines ; it will vary inversely 
as the third power of the wave-length, and will, therefore, be 3’2 for each centimeter 
in the violet. We have also to reduce the resolving power in the ratio of 10:12, 
as the beam is limited l)y a circular aperture. We have, therefore, finally in the 
violet 
II = 11-4 X 3’24/l’2 = 31, 
P = 4 X 31/21 = 6. 
2 T 2 
