The solar spectrograph is attached by clamps to the plate A l, the stellar spectrograph to the plate A2.
This has the effect of forming the image of the latter farther from the observer's eye, and so it becomes necessary to turn the handle of the rack-pinion V in such a way as to move the prisms P3 and P4 nearer to P2 till the lines of the stellar spectrograph are again sharply in focus.
Then if the prism P4 is cemented to P3, a sharp image of such lines of the solar spectrograph as are visible in the field of view will be seen in the eyepiece.
Therefore, if one-half of the cemented face of P4 is silvered, it becomes possible to view, side by side, one-half of the image of the solar spectrograph formed by 0 1 and one-half of the image of the stellar spectrograph formed by 02.
The narrow tongues of the silvered surface will now reflect corresponding parts of the star-spectrograph, and will obliterate corresponding parts of the solar spectrograph - as shown in figs.
If the stellar spectrograph is viewed in the focus of 0 2 and the converging rays are reflected by the prism P2 to P4, no image would be seen in the eyepiece, for the rays would pass out directly through the parallel glass plate which is formed by the cementing together of the prisms P 3 and P 4.
Then the lines of the stellar spectrogram would be seen in focus of the eyepiece and the image of the solar spectrograph would be obliterated.