Examination of Photographic Lenses at Kew. 429 



13. Definition at the Centre with the Largest Stop, G.I. Stop, 



No. - gives definition over the whole of a inch by 



inch plate. 



The system by which the defining power is measured consists 

 in ascertaining what is the thinnest black line of which the image 

 is just visible, the test being conducted in the following manner. 

 The test object consists of a thin straight strip of steel, about 

 O'l inch wide, and about an inch long; it is capable of being 

 rotated about an axis in the direction of its greatest length, thus, if 

 seen against a bright background, making it appear as a black line 

 of varying width ; when presented edgewise to the objective, it is so 

 thin that the image becomes invisible ; and there is an arc so gradu- 

 ated that the angle subtended by the two edges of the strip at the 

 lens can be at once read off, thus giving a measure of the apparent 

 thickness of the line. The test-object is placed as far as possible 

 from the lens in a darkened room (at Kew the accommodation in this 

 respect leaves much to be desired), and beyond it is a ground glass 

 screen illuminated by a lamp. 



In order to test the denning power of a lens in the centre of its 

 field, the focus is first very carefully adjusted on the ground glass, 

 and the test-object is then slowly revolved from the edge wise position, 

 where its image is invisible, until the first appearance of a dark line 

 can be seen against the bright background ; the angular width of the 

 line is read off, and is noted as a measure of the defining power of 

 the lens in the centre of its field. The light of the lamp is regulated 

 so that the image of the line can be seen as soon as possible. 



Besides measuring the defining power where the axis of the lens 

 cuts the focal surface, an observation is also made at a point repre- 

 senting the extreme corner of the plate of the size for which the lens 

 is being examined, that is, at a distance from the centre equal to half 

 the diagonal of the plate. As the object of this second test is to 

 measure the general definition over the whole plate, the focus is taken 

 at a position half-way between the point of observation and the axis 

 of the lens, this being the method generally adopted by practical pho- 

 tographers when desirous of getting the best general focus. It is 

 necessary, moreover, that the test-object should be so arranged that 

 the steel strip makes an angle of 45 with the horizon ; for, since the 

 diffusion of the image near the margin may be due to astigmatism, a 

 false impression of the defining power will be obtained if the image 

 of the dark line coincides in direction with either of the focal lines ; 

 whereas if it bisects the angle between them, as will then be the case, 

 there is no error in the result from this cause. The test is not, how- 

 ever, conducted in quite the same way as in the first instance ; the 

 test-object is set at a known angle, and the stops are slipped in one 



