OF SOME OE THE CHEMICAL ELEMENTS. 
141 
mator, seen through the train of prisms, remained approximative^ concentric with the 
object-glass of the telescope whilst the latter was moved through an extent of arc equal 
to the visible spectrum. All the pencils emerging from the last prism, therefore, with 
the exception of those of the extreme refrangible portion of the spectrum, are received 
nearly centrically on the object-glass of the telescope. The total deviation of the light 
in passing through the train of prisms is, for the ray D, about 198°. The interval from 
A to H corresponds to about 21° 14' of arc upon the brass scale. 
3. The measuring-part of the apparatus consists of an arc of brass, marked c in the 
figure, divided to intervals of 15". The distance traversed by the telescope in passing 
from one to the other of the components of the double sodium line D, is measured by 
five divisions of 15" each. These are read by a vernier. 
Attached to the telescope is a wire micrometer by Dollond. This records 60 parts 
of one revolution of the screw for the interval of the double sodium line. Twelve of 
these divisions of the micrometer, therefore, are equal to one division of the scale upon 
the arc of brass. The micrometer has a cross of strong wires placed at an angle of 45° 
nearly with the lines of the spectrum. The point of intersection of these wires may be 
brought upon the line to be measured by the micrometer screw, or by a screw attached 
to the arm carrying the telescope. For the most part the observations were read off 
from the scale, and the micrometer has been only occasionally employed in the verifica- 
tion of the measures of small intervals. The sexagesimal readings of the scale, giving 
five divisions to the interval of the double line D, have been reduced to a decimal form, 
the units of which are intervals of 15", and these are the numbers given in the Tables. 
An attempt was made to reduce the measures to the scale of Kirchhoff’s Tables, but 
the spectra are not found to be superposable on his. This is due, in great part, pro- 
bably to the prisms in his observations having been varied in their adjustment for 
different parts of the spectrum. The eyepieces are of the positive form of construction. 
One, giving the power of 15, is by Dollond ; the other, of about 35, is by Cook. 
4. The excellent performance of the apparatus is shown by the great distinctness and 
separation of the finer lines of the solar spectrum. All those mapped by Kirchhoff 
are easily seen, and many others in addition to these. The whole spectrum is very 
distinct. The numerous fine lines between a and A are well defined. So also are the 
groups of lines about and beyond G. H is seen, but with less distinctness. 
As, with the exception of the double potassium line near A, no lines have been 
observed less refrangible than a, the Maps and Tables commence with the line a of the 
solar spectrum and extend to H. 
The observations are probably a little less accurate and complete near the most 
refrangible limit. Owing to the feebleness of the illumination of this part of the 
spectrum, the slit has to be widened, and moreover, the cross wires being seen with 
difficulty, the bisection of a line exactly is less certain. 
5. For all the observations the spark of an induction coil has been employed. This 
coil has about fifteen miles of secondary wire, and was excited by a battery of Grove’s 
u 2 
