ApriL 28, 1904] 
NATURE 
609 
corresponding in width to this slit. The position of 
this second slit is, however, adjustable, and it can be 
made to coincide with any line in the solar spectrum. 
Thus any particular line can be completely isolated by 
this device. If now the solar image falling on the first 
slit be kept stationary, and the whole spectroscope be 
moved in a plane at right angles to the axis of the 
incident solar beam and in a direction at right angles 
to the length of the first slit, then the light that will 
pass through the second slit will be a succession of 
images in monochromatic light corresponding to the 
strips of the solar image which entered the first slit. 
By adjusting the second slit on, say, the K line of 
calcium, and placing a photographic sensitive plate 
fixed independently of but nearly in contact with the | 
surface of the second slit, then while the first slit is 
passing over the solar image, the second is moving over 
the film of the photographic plate at rest and allowing 
the monochromatic calcium K light to impress its 
successive images on the plate. The result is a 
picture of the sun in “‘K”’ light. Isolating different 
lines by means of the second slit, various mono- 
chromatic images of the sun in different ‘* lights ’’ can 
be secured. 
An instrument made on the above principle is the | 
ideal form to be used, but it can only be adopted when 
sunlight is thrown by means of a heliostat on to the 
lens which forms the image. It may be mentioned, by 
the way, that an ‘instrument on this principle has 
recently been erected at the Solar Physics Observatory, 
South Kensington. 
Prof. Hale wished, however, to employ his spectro- 
heliograph in connection with the great Yerkes re- 
fractor of 40 inches aperture, so that he was obliged 
to adopt another method, because the movement of the 
spectroheliograph, which is of considerable weight, 
across the solar image formed at the focus of this 
large lens would have made the telescope vibrate, and 
produced in consequence bad solar images. 
The method which he eventually adopted was to keep 
the whole spectroheliograph still in relation to the 
telescope itself, and to move the solar image uniformly 
across the first slit by means of a motor which actuated 
the declination slow motion of the telescope. Another 
difficulty then arose as regards the movement of the 
photographic holder, for this had to be made to travel 
across the second slit at the same uniform speed as 
the image over the first slit; this was finally overcome 
by connecting the plate holder directly with the declin- 
ation motor, thus moving them simultaneously. 
Although very similar in principle to the ordinary 
spectroscope, the spectroheliograph is different from 
it in many important details. In the first place both 
slits, instead of being straight as is usually the case 
in spectroscopes, are curved, and curved to a radius 
which is determined by the material of the prisms 
employed. Again, it is most convenient if the two 
tubes, carrying each a slit with its respective objective, 
and corresponding to the collimator and_ telescope 
tubes of the ordinary spectroscope, are arranged 
parallel to each other. This is accomplished by insert- 
ing between the collimator lens and the prisms (two | 
in this case) a plane reflector which can be so adjusted 
that the light, after being reflected and passing through 
the prisms, emerges parallel to the beam falling on | 
the reflector or to the collimator’s axis. i 
By the use of the reflector in this position, thus 
rendering the prisms clear of the optical axis of the | 
collimator, the instrument may be employed for | 
another line of solar research, because when greater | 
dispersion is required, as will be described further on, 
a grating may be inserted in its place. The optical | 
arrangement, as briefly described aboye, can be seen | 
from the accompanying illustration (Fig 1), which | 
| 
No. 1800, VOL. 69] 
shows this portion of the instrument alone. To 
indicate the path of the beam of light from the colli- 
mator objective to the second slit objective a white line 
has been drawn, the direction being indicated by the 
arrow head. 
For further information regarding the details of the 
construction of the instrument the reader may be 
referred to Prof. Hale’s account, but a few dimensions 
may be given here. Since the solar image formed 
by the forty-inch telescope measures seven inches in 
diameter, the two slits, both being supplied with the 
necessary means of adjustment, are eight inches long. 
The collimator and camera lenses are of the portrait 
type by Voigtlander, and are of equal aperture (63 
inches) and focal length. ‘These lenses are really too 
small for the large solar image dealt with, but Prof. 
Hale’s statement that the considerable cost of lenses 
of about ten inches, the required size, rendered their 
purchase difficult explains this defect. 
Passing over several important points that are vital 
to the efficient working of this instrument, which would 
Fic.t1.—Showing the optical arrargement of the Spectroheliograph. 
here take up too much space even to refer to at short 
length, such as a description of the movable plate- 
holder, the adjustment of the several parts of the 
spectroheliograph, the value of diaphragms to reduce 
diffuse and reflected light, the method of setting the 
second slit on any particular line in the spectrum, &c., 
we now come to describe some of the results which are 
the first fruits of this research. 
It has been stated above that if a line due to calcium 
be isolated by the second slit then we shall obtain a 
picture of the sun in calcium light; if a hydrogen or 
iron line be used, then a hydrogen or iron solar picture 
will be obtained. The lines which are the most easy 
to employ, and which give the best results, are those 
of H and K due to calcium. These lines in the solar 
spectrum (Fig. 2) are broad, of a composite structure, 
and are composed of three main parts, (1) a broad, 
| dark band, designated by Prof. Hale as H, or K,, 
(2) a comparatively narrow, bright line, lying at 
the centre of this band at points on the sun’s disc where 
the slit crosses hot masses of calcium vapour (H,, K,), 
