642 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and some of the curves have been calculated by Struve.* J. de Graaff 

 Hunter has now designed an apparatus adapted for use with the Beck 

 photographic lens-testing bench at the National Physical Laboratory. 

 The lens for test is held in the Beck bench so as to be free to turn 

 about a vertical axis through its back nodal point, thus permitting the 

 definition for oblique pencils to be readily examined. As an object, the 

 image of which is to be examined, is employed an " edge," forming 

 the boundary line between a half-bright, half-dark field. The " edge " 

 is placed at the focus of a collimator, so that it may be regarded as 

 virtually at an infinite distance from the optical system, say a photo- 

 graphic lens, to be examined ; the image formed by such lens is viewed 

 by a Microscope. In the focal plane of the eye-piece of the Microscope 

 (Ramsden) is placed a fine slit, parallel to the " edge," through which 

 alone the light passes. The slit is traversed across the field by a 

 micrometer screw. The edge which serves as object is cut on the 

 semi-circumference of a metal disk. Only a very small portion of the 

 edge is actually seen in the image, and this is straight to the order of 

 accuracy necessary. Over the other half of the circumference are sectors, 

 cut to such a depth that the inner radius is less than the radius of the 

 edge. The edge, at the part viewed, is uniformly illuminated from 

 behind. When this disk is rotated, over half the rotation there appears 

 in the Microscope field the blurred image of the edge ; over the other 

 half there is uniform illumination in the field, reduced only by 50 p.c. 

 owing to the interposition of the sectors. In general, at an appropriate 

 speed of rotation, there will be a flicker as passage is made from one half 

 of the circumference to the other. A second disk placed behind the first 

 gives the means of making a measurement. If the two disks be rotated 

 together, over the one half of the rotation is seen the blurred image of 

 the edge, the illumination being everywhere reduced by one-half owing 

 to the sectors ; over the other half of the rotation there is uniform illu- 

 mination, which, however, can be changed, in a measurable proportion, 

 by adjustment of the second disk relative to the first. 



The author describes the practical details, and the results are found 

 to be in complete agreement with the usual theory. 



Meyer's Search-stage ii. (Perquirator).f — A. Meyer describes this 

 auxiliary, which is made by the firm of Seibert in Wetzlar, and is 

 numbered 72 in their Catalogue.! It is really an improved form of 

 the same author's Search-stage i., designed as far back as 1901, and 

 numbered 71 in the same Catalogued The perquirator (fig. 107) is 

 essentially a mechanical stage with two mutually perpendicular move- 

 ments, andean be used as such an ordinary stage if the knob F (fig. 108) 

 is brought to the middle position, so that the toothed wheel actuating 

 the search-stage is put out of gear. The forward and backward move- 

 ment is by such rackwork as is used in mechanical stages. But here the 

 spindle-screw S, operating the lateral movement, is connected with an 

 arrangement consisting of two toothed wheels, by means of which a lever 

 H imparts a cross-movement to the preparation applied to the apparatus. 



* Wiedemann's Annalen, xvii. (1882) p. 1008. 



t Zeitschr. wiss. Mikrosk., xxvi. (1909) pp. 80-3 (2 figs.). 



X Catalogue, 1909. § Loc. cit. 



