59

SOLAR EYE PIECES.

The smallest sized speculum mentioned in this book, and any object glass above 2 inches aperture will concentrate sufficient heat to crack the coloured glass of an ordinary sun-cap, and thereby endanger the observer’s sight. A specially constructed- eye-piece is, therefore, absolutely necessary for continuous solar observation. One of the most simple methods of diminishing the sun’s light and heat is the use of a Barlow’s lens, silvered as the mirrors. A thin film should be deposited on each surface of the lens in preference to a single thicker film on one surface. These films are so transparent that when held up to the light they appear of an exquisite neutral tint, and when the lens thus coated is placed in its usual position in the draw-tube, and the Sun viewed through them, fine definition will be obtained, without any further protection.

Many observers use a diagonal eye-piece, in which one surface of the reflecting prism reflects sufficient light to secure perfect definition, whilst the excess of light and heat is transmitted through the prism.

We have lately constructed for Mr. Bessemer a solar telescope consisting of a 6-inch glass mirror, the front surface of which is polished to a parabolic curve, but unsilvered, and the back polished concave, so as not to give a second reflection, as it will present a transparent convex surface to the incidental light and will cause it to diverge. The cell is cut away so as to allow a large proportion of the light to pass through. The flat is also unsilvered, and being worked as an ordinary solar prism, also gives a single reflection; the back of the flat mount is also cut away to allow the light and heat to be still further diminished by passing through the transparent prism. With this instrument I have observed the Sun with ease without requiring any sun cap.

The late Mr. Dawes devised a solar eye-piece which bears his name; this consists of a diaphragm pierced with holes of various sizes placed in front of the eye-piece and capable of regulating either the amount of light or diameter of field. A cheap modification of this eye-piece has been lately introduced by Mr. Levander, in which a single aperture can be contracted or enlarged with great facility and exactness. A very good and safe plan of observing the Sun has been employed by Mr. Howlett: it consists in throwing the Sun’s image direct from the eye-piece upon a white disc. The further the disc is off, the greater will be the magnified picture. Several observers can thus, together, view the solar disc, the phenomena of which will be the more distinctly seen if the Sun’s rays are not allowed to enter the room, except through the telescope.

BARLOW’S LENS.

If in an achromatic object-glass the concave lens has a greater power than the convex, the resultant focus will be negative, that is,

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