V. Mr. IVaddel's Compound Microscope.
A very ingenious and useful compound microscope, represented in Plate CCCLXXVIL Fig. 23. Nos. 1, 2, and 3. has been constructed by Alexander \Vaddel, Esq. Leith, which possesses many advantages. It is founded on the properties of the astronomical telescope; which, as it inverts the objects, has a right-angled triangular prism A affixed to the outer end of it, at a certain point in the anterior focus of the object-glasses ; and by plac ing the instrument in a vertical, but somewhat inclined position, as in No. I, and the eye at the diopt•ic eye piece E, a picture will be seen within it of the external objects, erect, but reversed. If this instrument be plac ed in a horizontal position, as in No. 2, and another right angled triangular prism affixed to the outside of the eye piece at E, in a position perpendicular to the other prism, and the instrument so placed that external objects come into it from the observer's left hand side, then, by placing the eye at the last mentioned prism, and adjusting the sliding tubes to the sight, a picture of the external ob jects will be seen distinctly formed within the instru ment exactly as it is in nature, neither being inverted nor reversed ; and if this last prism have an opening in the lower side of its cover, as in the camera lucida, by a similar adjustment of the eye, the picture within the in strument will appear to he painted on a table under the prism, and may be copied by drawing with a pencil the outline, on a paper placed for that purpose, as is done in the camera lucida.
The way in which Mr. Waddel has commonly made use of this instrument as a compound microscope is, by placing it on a stencl in a vertical !Mt somewhat inclined position, as at first mentioned, with one of its prisms affixed to the object-end of it ; and when the whole of the tubes are drawn fully out, the length of the instrument from the extremity of the prism at one end, to tee outside of the eye-piece at the other, is from 9 to 10 inches ; which length has been found by the aid of two object-glasses of a compound focus, of about one and a halt inch, and a Huygentail eye-piece, with a double eye-glass, of a compound focus of about three fourths of an inch, to produce a magnifying power, in surface, of about 1020 times, and by applying to the outside of the said object prism an additional magnifier, a much greater power is obtained.
The largest tube of this instrument is less than two inches in diameter; and when the other three sliding tubes are pushed into it, with the prism attached to the object end of it, the whole length very little ex• ceeds 5 inches. Flom its particular construction, it produces, as an astronomical telescope, a magnifying power of ahcut two and a half times, and the field of view subtends an angle of about 30 degrees. It will be seen from this description of the instrument, that it not only combines the properties of a compound mic•o scope, but of a camera obscura, camera lucida, and diagonal mirror ; and that in the first and last of the three foregoing positions, it also becomes an excellent drawing instrument by the aid of a micrometer placed within it near the field-bar of the eye-piece.
An instrument of this construction has been found very useful in botanical and mineralogical pursuits, as objects may be viewed by it under different circumstan ces with the object-prism, by drawing out or pushing in the sliding tubes, which increases or diminishes the mag nifying power at pleasure, without the application of any additional magnifier.
As objects viewed by this instrument are reversed when only one prism is employed, and as certain imper fections arise from the increase of refraction and reflec tion, when two prisms are used, to render the objects neither inverted nor reversed, it occurred to Mr. Wad del, that some advantages might result from uniting the two prisms in one, as in Fig. 23 No. 3; which he ef fected by forming two right-angled triangular prisms out of one piece of glass, in such a way that the two re fleeting surfaces were at right angles to each other. When a prism of this construction is applied to the ob ject end of the instrument, with one half of it placed in the axis, and the other half without, and the instrument held in a vertical position, so as to admit external objects from the observer's left hand, then, by looking into it with the dioptric eye-piece, a picture will be formed as in nature, neither inverted nor reversed, which may be correctly copied by the aid of the micrometer before mentioned; and when at the; other en I, becomes a came ra lucida. Transparent objects may also be viewed by this instrument in the ordinary way, either with or with out the prism affixed to the objact end of it.