IN CONNEXION WITH THE SPECTRUM OF THE SUN. 
255 
The arrangements adopted will be easily gathered 
from the annexed woodcut (fig. 1) and the accompanying 
description. It is scarcely necessary to add that an 
important condition of this new method is that the 
object-glass of the collimator should be filled with 
light, and also that no light should be wasted. So 
long as these conditions obtain, conjugate foci and 
different lenses may be employed and the size of the 
image varied at pleasure, and still the brightness of the 
spectrum will be sufficient. 
The instruments with which the observations have 
been made are as follows 
A large spectroscope, a sister instrument to that used 
by Bunsen and Kirchiioff in their celebrated re- 
searches, and made by the same maker, Steinheil of 
Munich*. It is furnished with four prisms of flint 
glass. Three are of an angle of 45° and one of 60°. 
The general arrangements of the instruments are 
described by Kirchhoff in his memoir. 
In front of the slit plate is placed a lens throwing on 
the slit the image of the spark. 
A coil, made by Apps and giving a 4-inch spark. 
A large Leyden jar has also been occasionally used as a condenser on the secondary wire. 
Beneath the observing-telescope is placed a commutator, by which the current is 
controlled by the observer without changing his position. 
The window of my laboratory looks due south, and the collimator is placed in the 
same direction ; and when it became necessary to have the solar spectrum in the field, 
the light reflected from a heliostat placed outside the laboratory in direct prolongation 
of the line of collimation was thrown on to the lens and thus on to the slit, where the 
size and intensity of the images could be varied at pleasure by altering the position of 
the lens. 
When it was required to photograph a spectrum, the ordinary observing-telescope of 
the spectroscope was dismounted, and its place supplied by a telescope of 3f inches aper- 
ture and 49 inches focus. This was supported on the cast-iron table of the spectroscope 
at one end and at the other on a stand. The eyepiece and its mounting were removed, 
and against the end of the tube, thus left free, a small camera-box, holding a plate 4^ in. 
by 3^ in., was placed, and the photograph taken in the usual manner, the focus being 
obtained partly by careful observation with powerful magnifiers and partly by trial 
plates. 
* This spectroscope has been temporarily placed at my disposal by Professor Guru pan, of the Royal School 
of Mines, to whom my best thanks are due. 
A. Collimator. 
B. Observing Telescope. 
C. Spark. 
D. Lens. 
2 m 2 
