SCIENTIFIC SUMMAKY. 
203 
the instrument be mounted in this simple manner,* the observer must 
rest both elbows securely upon the table in order to keep the limb of the 
Sun precisely on the centre of the slit. A little practice is all that is 
necessary. The red line due to hydrogen produces a monochromatic image 
of the chromosphere which partly fills up the dark C line, and, if all the 
focussing is good, the little tongues that cover the outer surface — especially 
near the equatorial regions — can be distinctly seen with this instrument. To 
examine different parts of the limb the spectroscope is rotated, and the slit 
used tangentially. The angle of rotation is measured on a divided circle 
attached to the telescope by a small pointer fixed on the spectroscope. 
To save the eye fromthe glare when the full sunlight passes occasionally 
through the prisms, a diaphragm is placed in the focus of the eye-lens, 
so that all the spectrum is cut off except a little on either side of the 0 line. 
This answers perfectly. A little scale might, with advantage, be added, so 
as to measure approximately the length of the portion of the limb occupied 
by a prominence. 
The zero of the position circle may be obtained by turning the slit until 
the rotation of the earth causes a prominence to travel evenly along it. 
Owing, however, to the want of stability in the mounting, I prefer to 
measure the position of the prominences from the vertex of the disk, the 
zero being determined by placing the slit several times horizontal and 
vertical by estimation. 
“ The height of a prominence above the upper surface of the chromosphere 
may be measured by opening or closing the slit until it just contains the 
prominence. The value of a revolution of the micrometer-screw for open- 
ing the slit may be found accurately by actual measurement, and turned 
into seconds of arc, for the radius equal to the focal length of the object- 
glass, by simple proportion. A focal length of 100 inches requires the slit 
to be open to 0-0485 inches to subtend 100 seconds of arc. 
‘‘ A quicker way, and quite as accurate, is to turn the spectroscope round 
until the top of the prominence and some other part of the surface of the 
chromosphere enter the field together. The angle through which the 
spectroscope is turned -will give the perpendicular height of the prominence 
as in the following table, which is sufficiently accurate all the year round. 
Angle. 
Height. 
Angle. 
Height. 
Angle. 
Height. 
10 
15 
o 
22 
77 ■ 
34 
199 
11 
19 
23 
84 
35 
212 
12 
22 
24 
92 
36 
228 
13 
26 
25 
99 
37 
244 
14 
30 
26 
109 
38 
261 
15 
34 
27 
119 
39 
277 . 
16 
39 
28 
129 
40 
293 
17 
45 
29 
139 
41 
312 
18 
51 
30 
149 
42 
333 
19 
56 
31 
161 
43 
355 
20 
62 
32 
.7 4 
44 
379 
21 
69 
33 
187 
65 
404 
Viz. on a pillar-and-claw stand. 
