OS CATADIOPTRIC BURNING INSTRUMENTS.
In order to construct a burning instrument which shall in a great measure, be unlimited in its power, we must combine the principles both of reflection and refraction. We are not aware that any instrument of this kind has ever been proposed ; and we are the more surprised at this, as the proper combination of lenses and mirrors must naturally suggest itself to any one who considers the limits which are set to the construction of single lenses, and the disadvantages either of a theoretical or a practical nature, to which they are liable.
This instrument, which has been proposed by Dr Brewster, and which may be properly called a burning sphere, from the arrangement of the lenses, is repre sented in Plate CVI. Fig. 5. which is merely a section of the sphere, and represents only five of the lenses, and four of the mirrors. The lenses A, B, C, D, E, which may be of any diameter and focal length, are so placed in the spherical surface AMN, that their principal foci exactly coincide in the point F. If any of the lenses have a different focal length from the rest, the coinci dence of its focus with that of the other may be easily effected by varying its distance from F. The whole sphe rical surface, whose section is AMN, except a small opening for admitting the object to be fused, may be co vered with lenses, having all their foci coincident at F ; though it will, perhaps, be more convenient to have the posterior part MN without lenses, and occupied by a mirror of nearly the same radius FA as the sphere. The object of this mirror is to throw back upon the object at F the light that passes by it, without producing any ef fect. Each of the lenses, except the lens A, is furnish ed with a plane glass mirror, which may be either fixed to the general frame of the sphere, or placed upon a se parate stand. When this combination is completed, the sphere is exposed to the sun, so that its rays may fall at right angles upon the lens A, which will, of course, concentrate them at F, and produce a pretty intense heat. The plane mirror PQ, when properly adjusted,
will reflect the sun's light perpendicularly upon the lens B, by which it will be refracted accurately to the focus F, and produce a degree of heat fully one half of what was produced by the direct refracted rays of the sun through the lens A. A similar effect will be produced by the mirror RS and lens D, the mirror TU and lens C, the mirror VW and lens E, and by all the other mir rors and lenses which are not seen in the section. The effect may be still farther increased by the addition of a large lens at XX. As the angle which the surface of each mirror forms with the axis of its corresponding lens is a constant quantity, the mirrors may he all fixed to the general frame of the sphere, and therefore the only ad justment which the instrument will require, is to keep the axis of the lens A parallel to the direction of the so lar rays.
In order to estimate the advantages of this construc tion, let us compare its effects with those of a solid lens, which exposes the same area of glass to the incident rays.
I. In the burning sphere, almost the only diminution of light is that which arises from reflection by the plane mirrors, and which may be estimated pretty accurately at one half of the incident light ; but this loss can bt, amply compensated by adding a few more lenses.
2. In the solid lens, a great diminution of light arises from the thickness of the central portions, and from the obliquity of the parts at the circumference ; which, we conceive, will be fully equal to the light lost by reflec tion in the burning sphere.
3. In the burning sphere, the lenses may be obtained of much purer glass than can be got for a solid lens ; and therefore, cceteris paribus, they will transmit more light.
4. Owing to the small size of each lens in the burning sphere, the diminution of effect arising both from sphe rical aberration and from the aberration of colour, will be very much less than in the solid lens.