## A Lecture on Lenses |

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allel angle astigmation centres of curvature chromatic aberration circle of aberration colored images coma combination of lenses concave conjugated foci converging lenses cross the axis curvature of field curved side diameter diamond diaphragm different colored rays distance distortion diverging lenses diverging rays Dollond double convex lens enter the lens equal radii flint glass focal length fracted Franklin Institute Fraunhofer hole human eye inches incident rays index of refraction infinite number large photographic objective larger aperture lens of equal liquely longitudinal aberration luminous point marginal rays Meniscus move the screen nearly negative lens nodal point normal optical centre opticians pin-hole camera plane surface plano convex lens positive lens principal axis principal focus radiating point ray of light rays coming rays cross rays passing rect red and violet red rays retina sharp image single lens situated spherical surfaces straight line suppose tion violet ray white light wide apertures

### Popular passages

Page 2 - THE SIZE OF THE IMAGE DEPENDS UPON THE DISTANCE OF THE OBJECT FROM THE CENTER OF THE LENS.

Page 20 - We come now to the last of the more important aberrations, that is the astigmation, a word coming from the Greek, meaning : not coming to one point. If we focus a well defined, round object, situated in the axis of a lens of a wide aperture, on a screen, we find the image round, even if we move the screen in and out of the focus, the image will get only less sharp ; but if we turn the lens sideways, so as to get the image of the same object formed by pencils oblique to the axis, then we will observe...

Page 11 - Herschel himself has rectified in his memoirs. We now come to the most important method of correcting spherical aberration, that is, by a second lens of opposite character. Suppose we want to correct the spherical aberration of the positive lens, L, (Fig. 14) along its axis. //' is the longitudinal spherical aberration of the rays AB, parallel to the axis A, at the margin of the lens, and B near the centre of the lens L. If we combine this lens with a convergent negative lens, M, it is not difficult...

Page 3 - As the normals n and n' are parallel, so must the incident and refracted ray be after leaving the glass. Now let us see another case, where the two surfaces are not parallel, but form an angle with each other. Such a medium is called a prism. R o (Fig. 4) is an incident ray ; the ray is refracted towards the normal...

Page 10 - ... microscope, the image with all its errors is magnified by an eye-piece. Let us now see what means we have to reduce, correct, or destroy the spherical aberration. The most simple way is by the use of a diaphragm. A diaphragm is a non-transparent plate, commonly made of metal, perforated in centre.

Page 20 - AS, and the point R, which will cut the lens in its diameter CD. Let us lay another plane through the point R, at a right angle to the former, and which will cut the lens in its diameter E F. If we draw the line R p through the optical centre of the lens, a ray following it would not be refracted, as we have seen before, and constitutes a secondary axis. R...

Page 7 - If we now prolong R and R in their first direction, they will meet at a point P, the one nodal point, or the centre of admission, and if the emerging rays are also prolonged, they will converge to a point P', the other nodal point, or the centre of emission. We recollect that...

Page 10 - ... aperture, and the same equivalent focus of the single lens. We have seen before, that two lenses of the same aperture, but their focal length, as 1 to '2 to each other, the longer one has only one-fourth of the spherical aberration of the shorter one. Lens M (Fig. 13) has its focus at/. The lenses L and N are of the same aperture as M, but each has twice the focal length of the lens M ; therefore, each has only onefourth the spherical aberration of M, but L and N together have the same focal...

Page 9 - This aberration is called positive. If converging rays RR and R'R' (Fig. 11), which we suppose would be collected in the point o, fall on a concave lens, the marginal rays RR are refracted stronger than the more central ones R'R', consequently RR will cross the axis farther from the lens, at/', than the more central ones, R'R', which cross the axis at /. In this case the spherical aberration is of the opposite character, and is called negative aberration. It is evident from the foregoing that spherical...

Page 11 - By this method the spherical aberration can not only be corrected, but the marginal rays can be made to cross the axis farther from the lens than the central ones ; in this case the lens is called over-corrected, while if not enough corrected, it is called under-corrected.