Fig. 215.—Comparison between Kirchhoff’s Map and Rutherfurd’s Photograph.
This magnificent spectrum extends from the green part of the spectrum right into that part of the spectrum called the ultra-violet. Of course it had to be put together from different pictures, because there is a different length of exposure required for the different parts; the exposure of any particular part of the spectrum must be varied according to the amount of chemical intensity in that part. If the line G was exposed, say for fifteen seconds, the spectrum near the line F would require to be exposed for eight minutes, and at the line H, which is further away from the luminous part of the spectrum than G, there the exposure requisite would be two or three minutes.
Fig. 216.—Arrangement for Photographically Determining the Coincidence of Solar and Metallic Lines.
Fig. 217.—Telespectroscope with Camera for obtaining Photographs of the Solar Prominences.
In order to obtain a photograph of the average solar spectrum, the camera replaces the observing telescope, and a heliostat is used, as in the ordinary way. The beam, however, should be sent through an opera-glass in order to condense it, and thereby to render the exposure as short as possible.
Further, if an electric lamp be mounted as shown in Fig. [216], observations, similar to those originally made by Kirchhoff, of the coincidence on the various metallic lines with the Fraunhofer ones, can be permanently recorded on the photographic plate. The lens between the lamp and the heliostat is for the purpose of throwing an image of the sun between the carbon poles. The lens between the lamp and spectroscope then focuses both the poles and the image of the sun on to the slit. The spectrum of the sun is first obtained by uncovering a small part of the slit and allowing the image of the sun to fall on this uncovered portion, the lamp not being in action. When this has been done the light of the sun is shut off. The metal to be studied is placed in the lower pole; the adjacent portion of the slit is uncovered, that at first used being closed in the process. The current is then passed to render the metal incandescent. After the proper exposure the plate is developed and the spectra are seen side by side. Fig. [187] is a woodcut of a plate so obtained.
If the spectrum of any special part of the sun, or the prominences, has to be photographed, then either a siderostat must be employed, or a camera is adjusted to the telespectroscope, as shown in Fig. [217].
For the stars, of course, much smaller dispersion must be used, but the method is the same; and what has already been said by way of precaution about the observation of stellar spectra applies equally to the attempt to obtain spectrum photographs of these distant suns.