THE ACHROMATIZED MENISCUS, OR COMMON VIEW LENS.
This lens is composed of two, which are cemented together with Canada balsam. The first is of flint glass, and double concave ; the second of crown glass and double convex. The concavity of the front surface of the flint lens is very slight ; this surface may even be quite plane. The radius of the inner surface is the same for both lenses, as they are cemented together. When this lens is used a stop is placed at a suitable distance in front of it.
But a lens of the meniscus form may be achromatized in two different ways. When the flint glass receives the incident rays the construe tion is that described above ; but the crown glass may be placed so as to receive the incident rays. In this case the crown lens is a meniscus, and the flint lens concavo-convex. The two forms are exhibited in the following figs.:—the letters C,F denoting crown and flint.
When the focal length of the achromatized lens, the radius of the front surface, and the materials used are the same, the focal length of the crown lenses are the same in both cases, and also the focal lengths of the flint lenses ; the radius of the posterior surface is also the same in both cases. The first form has been in use for a number of years, but the second has been lately introduced and is said to give less spherical aberration.
The object of this view lens is to give a good image on a flat sur face, pretty equally illuminated, and well defined, when an angular field of view of from to is included ; the intensity of light in the image being a matter of secondary importance, and the spherical aberration being sufficiently reduced by the use of a small diaphragm. In other words, the object of this lens is to give a uniformly good picture, including a wide angular field, without particular reference being made to the amount of light admitted, or the time of exposure of the photograph.
The principle of this form of lens will be best understood by discussing, in the first place, the case of a single plano-convex lens, with a stop in front.
Let AB be a plano-convex lens presented to objects at such a distance that pencils from them may be considered cylindrical ; and suppose an oblique cylinder of rays MA NB from one of these dis tant objects not situated upon the axis of the lens, to be incident upon the plane side of it. Every ray of this large cylindrical pencil then suffers the same amount of refraction on entering the lens, and the pencil within the glass is a cylinder having its rays parallel to mA or nB. We have now done with the plane surface, and the case is simply that of a cylindrical pencil within a sphere about to emerge. It remains to be seen what becomes of it.
One of its rays DE, if produced backwards, passes through the centre C of the spherical surface. This ray does not suffer deviation at emergence, but proceeds in the same straight line, D E, produced to F. Rays emergmg at an equal distance from E cut the line DF in the same point and have equal aberration, and the caustic stuface is symmetrical with respect to EF, which is therefore its axis • and EF is equal to the principal focal length of the lens for a 'direct pencil.
It appears therefore that EF is a constant quantity, not dependent on the obliquity of the pencil ; CE is also a constant quantity, being the radius of the spherical surface AB ; therefore CF is a constant quantity.
Hence it follows that when a plano-convex lens is directed to ex tremely distant objects, the image lies upon a spherical surface which has the same centre as the posterior surface of the lens.
This will perhaps be understood better by reference to the follow ing figure :— AB is the lens ; C, the centre of the sphere AB ; P, Q, R, fixed stars emitting cylindrical pencils of light ; QC q, the axis of the lens ; and p, q, r, the images of the stars, which lie on the spherical surface shown by the dotted line, and the centre of which is C.