John S Anderson

A criticism of the Nodal slide as an aid in testing photographic lenses

The nodal slide is only convenient for the examination of lenses over their entire field when these are of normal type. Collimators and lenses should in general be directed to pass through all useful light as symmetrically about their axes as possible. Failure to secure this condition may lead to an underestimate of the excellence of good lenses as well as an underestimate of the gravity of the faults present in bad lenses. The nodal slide may with advantage be supplemented by suitable linkages, which enable all the special conveniences of the nodal slide to be retained. Some of the possible modifications are discussed.

A magnification method of measuring focal length

The method described depends mainly on the determination of the distance between the two positions which the lens occupies corresponding to the reciprocal magnifications - m and - I/m. Details of the method as applied to the measurement of the focal length of a small achromatic lens are given. The necessary precautions to be taken in making the measurements and the corrections to be applied when the curvature of field is taken into account are discussed.

The measurement of distortion in a gauge projection lens

The distortion of a lens working at finite magnification is deduced from measurements of the variation of magnification of an object as the latter is traversed across the object field or the image field. It is found that as regards attainable accuracy there is little to choose between the alternative methods of measuring an enlarged image of an object in the object field or a reduced image of an object in the image field. The technique of the method as applied to a new gauge projection lens is described.

An application of the photo-electric cell to a comparison of the efficiencies of photographic shutters

The paper describes a method of comparing the efficiencies of shutters, incorporating photo-electric measurements of the total amounts of light passed by the shutters. Details of an application of the method to a new type of shutter are given.

Photographs of reflexion caustics

The photographs are of caustics obtained by light reflected successively at two surfaces of a photographic lens.

Two new methods of measuring the internal diameters of transparent tubes

Two new methods of measuring the internal diameters of glass and other transparent tubes are described. The first consists in immersing the tube to be tested in a suitable liquid and varying the wave length of the illuminating beam until equality of refractive index is obtained. The system being now homogeneous, the diameter can be measured directly. Information is given with regard to the most suitable liquids to employ and the paper contains a number of practical hints which will facilitate the carrying out of the test. The second method consists in taking an X-ray shadow photograph of the tube and measuring the dimensions of the image formed. Between the two methods it is possible to measure accurately the internal diameter of practically any type of transparent tube, whether sealed or open, no matter what the outside shape of the tube may be.

The measurement of the internal diameters of transparent tubes. Part I

In order to estimate the internal diameter of a sealed tube the only method which appears to have been described is that in which the apparent internal diameter is measured when the tube is illuminated with monochromatic light. The method is not directly applicable when the refractive index of the material of the tube is not known. Two modifications which obviate this difficulty are described in this preliminary note. The description of two other more convenient methods, applicable to the cases of sealed or open tubes, is reserved for a subsequent paper.

Notes on photographic shutter testing

The paper briefly describes a number of the more important methods of measuring shutter speeds and in particular the method employed at the National Physical Laboratory. Details of a number of recent modifications are given and a method for determining data connected with the motion of the blind of a focal plane shutter is described.

A simple differential refractometer for liquids

A number of sensitive differential methods for liquids are briefly discussed and a simple new method, which is applicable to a number of problems, is described.