Dr. (Sir David) Brewster, in his Treatise on New Philosophical Triatruinente; recommends the employment of sulphuric acid and oil of ramie for the composition of fluid lenses, by which the secondary spectrum may be destroyed ; the acid being, of all known substances, that which exerts the greatest, and the oil that which exerts the least, action on the green-coloured rays. The correction of the chromatic dispersion by means of fluids was also attempted by Mr. Barlow, as noticed in the following article. But probably from imperfection in the forms nf the glasses, the images of objects were found to be not well defined ; and the construction, In consequence, has not been adopted.
The image formed by the great 'speculum of a reflecting telescope Is free from the Inconvenience* attending the chromatic aberration of light ; for the angles of incidence being equal to those of reflection, in any pencil coming from a point in an object, all tho rays will converge to one peiut at the place where the image is formed. Ii the surface of the speculum were that which is formed by the revolution of a parabola about its axis, then all the rays In any pencil proceeding from a very remote object, as one of the celestial bodies, and being Incident on the speculum in a direction parallel to the axis, would, by the nature of the parabola, converge accurately In the focus of tho curve; and on this account, an effort la always made to give to the reflecting surface of the speculum a paraboloidal figure. The advantage does not, how ever, hold good with the pencils which fall on the mirror in directions oblique to the axis ; and therefore that figure is of leas importance, when the telescope is to be used for viewing terrestrial objects, than when it is to be employed for astronomical purposes; for then, on account of the great distance of the objects, the several pencils; of light fall on the mirror with a very small obliquity to its axis.
The telescope invented by Galileo consisted of one convex lens, A n, fig. ti, and of a concave lens, c n; the distance between them being equal to the difference between the focal lengths of the two lenses. In this instrument, if the object o P were so remote that the rays in each pencil of light might be considered as parallel to one Another, there would be formed at its principal focus an inverted Image (o p) of that object by the union of the rays in each pencil in one point ; then the concave lens being placed between A s and that image, in such it situation that its principal focus may coincide with the place of that image, the rays in each pencil will, by the refracting power of the lens, be made to emerge parallel to one another ; and in this case, by the optical properties of the eye, distinct vision is obtained.
The line 0 x o is the axis of the pencil of light from o; and, as this passes through the centre x of the lens A B without refraction, the anglo zxo is equal to half the angle under which OP would be observed by an eye at x when no telescope is interposed, while parallel to Y o being the direction of a ray in that axis after refraction in c n) z Y 0 is half the angle under which o P is seen in the telescope: the ratio between these angles in therefore the measure of the magnifying power of the telescope ; and since the angles are to one another as Y z is to X z, nearly, it follows that X nearly expresses the magnifying power.
Yz This is the construction of what is called an opera glass ; and the Galilean telescope is now used chiefly for viewing objects within n theatre, or an apartment, since if considerable magnifying power were given to it the extent of the field of view would be very small.
A simple telescope may also be constructed by means of two convex lenses, fig. 6, which are placed at a distance from one another equal to the sum of their focal lengths. For the image being formed at the focus 2, of the lens A n, which is nearest to the object, as in the Galilean telescope, and being supposed to be a plane surface, the light also being supposed to be homogeneous, the rays of each pencil, after crossing at the focus and proceeding from thence In a divergent state, on being allowed to full upon the surface of the second lens, c e, may be refracted in the latter so as to pans out from thence in parallel directious; and consequently distinct vision of the object may be obtained by an eye situated so as to receive the pencils.
If so be the direction of the axis of a pencil of light coming from 0, one extremity of the object o r, which is 'supposed to be so remote that all the rays in each pencil may be considered an parallel to one another • then the angle z x o is half the angle under which the object o r would be seen by an eye at x without a telescope, while the rays of that pencil entering the. eye at E in the direction DE, which is parallel to o v, the angle z ye is half the angle under which the same object is seen when viewed through the telescope. Now these angles are to one another nearly as z to z x; therefore will express nearly the 2 Y magnifying power of the instrument. As the pencils of light from o and r cross the axis of the eye at E before they are united on the retina, the image of the object 0 P is formed in the eye in a position contrary to that which is formed when the object is viewed without the telescope ; therefore, on looking through the latter, the object o r appears to be inverted.