Russel Wallace : Alfred Russell Wallace (sic)
Curved Specula of Great Size, With a Few Remarks
on Fixed Telescopes (S712a: 1843/2006)
We should then have a perfect plane mirror of any size to reflect the rays to the concave speculum and thus one great difficulty of using telescopes of large size fixed, would be got over.
Having however obtained a plane mirror it appears not impossible that we may also produce the concave one without going through the tedious, uncertain, and expensive operations of grinding and polishing. It is evident that a plane mirror produced as before described would be both perfectly uniform in its thickness and perfectly homogenous in its structure. The writer of this was much interested some time ago with Mr. Nasmyth's method of producing concave reflectors by extracting the air from a chamber behind a circular plate of glass and allowing the pressure of the atmosphere to produce the required curvature. He has not been able to learn why this has not succeeded or been brought into operation, but supposes it must be from the difficulty of procuring plates of glass sufficiently uniform in thickness, homogenous in structure, and truly plane in both surfaces without all of which it is evident a uniform curve would not be produced. Mr. Nasmyth conceives that the curve would be between a catenary and circle, but does it not appear more likely that it would be truly parabolic, the pressure of the atmosphere being similar to an infinite number of perpendicular pressures. If so it appears difficult to imagine how a perfectly plane homogenous body of uniform thickness, placed under such a pressure could take any other than a true parabolic form. If it did there would be a curved mirror formed superior probably to any yet made. It also seems probable that the metal after being exposed to the pressure, would on its being removed retain a portion of its curvature sufficient for a long focus and thus obviate the inconvenience of having to keep the atmospheric pressure perfectly uniform in order to preserve the same focal length.
In fixing a telescope for the purpose of using a moveable plane Speculum to reflect the objects to the curved one, it appears evident that that position must be best which requires the plane mirror to be of the least possible size. To effect this the tube must be directed to a point midway between the Pole and a few degrees above the Southern Horizon or in this Country about 20° South of the Zenith, (on the supposition that it is not required to view objects more than a few degrees below the Pole) the Great curved Speculum being at the top and the plane mirror and observer at the bottom; In this situation the plane mirror need be only 1/7th larger than the Speculum to reflect rays to the whole of its surface from objects least favourably situated, while if directed toward the Pole with the Speculum at the top the plane mirror must be nearly twice the diameter and with the Speculum at bottom, no dimensions of plane mirror would reflect light from objects near the Pole to the whole surface of the Speculum.
The plane mirror might be made to keep an object in the field of view with great facility in whatever direction the telescope were placed, by fixing it in the manner of an equatorial so as to be elevated to the proper angle and moved round on a polar axis.
The writer has thus concisely expressed his ideas on these subjects in the hope that if they are at all novel, some one may be found to put them to the test of experiment and ascertain whether these methods of increasing the power of the Telescope are capable of practical application.
Copyright: Alfred Russel Wallace Literary Estate.