Art. XII.—On a “Direct-vision Solar” Eye-piece for large Telescopes.

By H. Skey.

[Read before the Otago Institute, 12th October, 1875.]

When coloured glasses are placed before the eye-piece of telescopes in solar observations, they intercept a certain portion of the heat, but this can only be by their absorbing it; in absorbing the heat they necessarily become heated, and when used with telescopes of greater aperture than two inches, they are liable to fuse or crack, thus endangering the eye-sight of the observer.

The screens here exhibited were devised originally for the purpose of observing the Transit of Venus, in December last.

In their construction, advantage is taken of the transparency of thin films of the metals, a film of silver being precipitated on glass, by Liebig's, or other methods, and then guarded by another glass. These films are chosen, so that part thereof is nearly transparent, and the other part nearly opaque, and by a sliding method, a suitable part can be brought over the field of view of the telescope so as to be adapted to the varying intensity of light, arising from clouds, etc. One of these films is on plane glass; the other is on a convex glass, which is guarded by a concave one. These dark screens appear to give excellent definition, and, being of a neutral tint, they allow of seeing the sun as a white globe on a black ground. In using them, the mirror side is turned towards the sun, and “light and heat being reflected in sensibly equal proportions,” their excess is reflected back, while sufficient light is transmitted through the film to admit of proper illumination, after the telescope has magnified the image. Testing these films for safety, under a large condenser, they resisted the heat, while colored glasses, even if only lightly colored, were instantly cracked.

This method has an advantage over the “Diagonal mirror” in admitting of direct vision, moreover, the liability of losing an observation at the critical instant is lessened, for by merely sliding the screen, it can be adjusted to the varying illumination without removing the eye.