EEV. T. E. EOBINSON ON SPECTEA OF ELECTEIC LIGHT. 
943 
formed by interpolation a Table of reduction to the latter, by means of which all are 
given on the same scale. The two glasses are so nearly similar, that it was not neces- 
sary to go heyond second differences. Even this prism leaves several bands unresolved, 
though often giving a suspicion of their compound character, which in some recent 
instances I have verified by combining two 60®-prisms of the bisulphuret. 
It is an object of great interest to ascertain whether there be any special relations 
between the wave-lengths of these luminous bands. In aid of this I subjoin a Table 
giving the value of X for every five minutes of Merz’s deviations within the range of my 
observations. It has been computed hy a very simple form of interpolation given by 
Professor Stokes*. Assuming and taking /a and ~ for any two of Feaun- 
A. 
hoeer’s Lines, we get, for any intermediate p, the \ simply hy proportional parts. This 
is so accurate, that it gives correctly one of the intermediate lines by taking the double 
interval, as D from C and E ; even H from F and G. 
' P - 
X. 
AX. 
A^X. 
X. 
AX. 
A2X. 
P - 
X. 
AX. 
A*X. 
32 35 
40 
45 
50 
55 
33 0 
5 
10 
15 
20 
25 
2493 
2405 
2326 
2257 
2194 
2136 
2083 
2034 
1989 
1948 
1909 
-88 
-79 
-69 
-63 
-58 
— 53 
-49 
-45 
— 41 
-39 
+ 9 
+ 10 i 
+ ^ i 
+ 5 
+ 5 1 
+ 4 : 
+ 4 : 
+ 4 j 
+ 2 1 
+ 2 j 
! 
' 33 25 
30 
35 
40 
45 
50 
55 
34 0 
5 
10 
15 
1909 
1872 
1837 
1805 
1774 
1745 
1718 
1692 
1667 
1643 
1620 
-37 
— 35 
— 32 
— 31 
-29 
-27 
-26 
— 25 
— 24 
— 23 
j 
+ 2 
+ 2 
+ 3 ! 
+ 1 1 
+ 2 
+ 2 
+ 1 
+ 1 
+ 1 
+ 1 i 
+ ^ 1 
0 / 
34 15 
20 
25 
30 
35 
40 
45 
50 
55 
35 0 
1620 
1599 
1580 
1562 
1544 
1526 
1509 
1493 
1478 
1464 
—21 
-19 
— 18 
-18 
— 18 
-17 
-16 
— 15 
— 14 
+ 1 
+ 2 
n- 1 
+ 0 
+ 0 
+ ^ 
+ 1 
+ 1 
+ 1 
Theodolite . — For the use of this instrument I am indebted to Mr. Grubb, who made 
it many years since to determine the ybs, of the glasses for his object-glasses. It is of 
simple and very firm construction. A strong brass disk, supported by three screws, has 
on its upper surface a circle 9 ‘5 inches diameter graduated to half degrees, and carries 
laterally the supports of a collimating telescope 9-5 inches focus and I inch aperture, 
which is provided with an adjustable slit. Above the disk turns a brass plate bearing 
two verniers in the plane of the divisions (which read to minutes), and supporting the 
telescope, with a triple object-glass 7 inches focus and 0'9 inch aperture. Below the 
disk turns another circle, similarly divided on its cylindric surface ; its axis rises through 
that of the upper plate, and carries a table 3 inches diameter, which bears the prism. 
The axis of the prism is adjusted by observing the images of the slit refiected from its 
surfaces, which also give its angle by means of the lower circle. Some of these matters 
require a few remarks. 
I. The telescope (and the collimator also), though sensibly achromatic on a day-object 
* Eeport of tlie British Association, 1849, Trans, of Sections, p. 11. 
