THE TESTING OF LENSES A lens should be well polished, and free from bubbles and other defects in the These faults are easily seen in a a dark if the examiner's eyes are screened. Defects of definition are more important, and not so simply dealt with. the tests applied to a lens should never be omitted its trial in actual use, but there should be added tests more severe than any to which it will be subjected in use and such as to indicate the and cause of any faults present. It must suffice to deal here with telescope and microscope objectives. Spectacle lenses, condensers and lenses for and eye pieces are made in and sold without any test at all made except to reveal such faults as be readily per ceived by a where manufacture is carefully controlled it is rare that any such lens that is not amply for the is put on the market.
A test-object for telescope objectives is a star, or an arti ficial star produced by a distant ball of black or a bulb of mercury, in which the sun is reflected. Actually, if the wave theory of be taken into account, calculation shows that the formed by a telescope lens is not a point, but a disc of finite diameter surrounded by of rapidly intensity (G. Airy, "Diffraction at a Circular Object Glass," Trans. Camb. Phil. Soc. 5, see also LIGHT) . Examination of the with a lens of power confirms the result of calcula tion. William Halley in knew that the of a star had a size than was attributable to the star alone, and Herschel knew of the referred to before any explanation of them was (J. F. W. Herschel, Encyc. Metropolitana, Examination of these particularly as they appear a little within and without focus, can yield with experience most valuable information any faults that may be and this method of test has been applied with excellent results by Dennis Taylor (The Adjustment and Testing of Telescope Objec tives, York, Another test which is informative is that of Foucault (Comptes Rendus, "Memories," Ann. de l'Observatoire de Paris, adopted later for other purposes by Toepler (Pogg. Ann. 1866-68). The eye placed in the focus of a star (or artificial star) sees the object uniformly illuminated. In the Foucault
test a knife passed slowly across the focus from to left makes the illumination disappear over the whole area of the object simultaneously if, and only if, the rays all pass accu rately the focus ; this affords direct evidence of the course of the rays from different parts of the object Still fuller of the course of the rays is to be had from Hartmann's method Zeits. f. Instru mentenk., In this, a pierced with holes is placed close to the object and the of a star within and without focus. Such consist of dots, each of which corresponds to one of the apertures in the and from the distances apart of the dots in the two the course taken by the rays from the parts of the object can be found. accurate results have been obtained by this method but it is very laborious. Methods which are of value in special cases have been developed by Waetzmann, founded on the Jamin Refractometer (E. Bratke, Ober die Waetz mannsche Interferenzmethode zur optischen Sys teme, Zeitschr. f. Phys. 5924), Ronchi (Vasco Ronchi, Ober die Schattenstreifen zum Studien der Lichtwellen Zeistchr. fur In strumentenkunde, and Lenouvel (M. L. Lenouvel, Revue d'Optique 3 pp. 2 II and 315. These last two derived from Foucault's test.
In a different from the above are the methods of examination founded by Twyman and Green (F. Twyman and A. Green, Brit. Pat. and 213,274/23, F. Twyman, Brit. Pat. on the interferometer of Michelson. The method is applicable to object of every microscope objectives and even complete lens systems—and the information is in terms of aberrations of wave-front pro duced by the lens, or if desired, by simple transformations (J. W. Perry, "The Determination of Aberrations, etc." Trans. Opt. Soc. in terms of the convention in which almost all practical lens computations are carried out ; the test is of accuracy, can be carried out very rapidly and can be recorded in a in which the errors due to each area of the lens aper ture are represented. Those who have adopted it as a means of their lens production have found it to yield readily information more complete and useful than that obtainable in any other way.