im 

 ca 





1892.] Examination of Photographic Objectives at Kew. 3 



whole of the plate. With reference to this last item, it is shown how 

 if a larger stop than the one here named is used, the illumination 

 will fall off very rapidly towards the edges of the plate, whereas 

 there will be no improvement in this respect by using a smaller stop. 



(11.) Principal Focal Length. This is found by revolving the 

 camera through a known angle, and measuring the movement of the 

 image of a distant object on the ground glass ; with the testing 



mera it is so arranged that an angular movement can be given with 

 great ease and accuracy, and that the angle is such that half the focal 

 length is directly read off on a scale on the ground glass. The 

 observation is made, and the focus adjusted, when the image is at a 

 point some 14 degrees from the axis of the objective, and the effect 

 of distortion and curvature of the field is discussed ; it is proved that 

 the focal length thus obtained, even though it may not be identical 

 with the principal focal length as measured on the axis, is never- 

 theless what the photographer in reality wants to ascertain. 



(12.) Curvature of the Field. The amount of movement of the 

 ground glass to adjust the focus at different parts of the field is 

 measured, and the results are recorded to show the curvature. A 

 table is given by which it can, be seen what is the largest stop which 

 can be used so as to produce negatives up to a certain standard of 

 excellence in definition. 



(13.) Distortion. This test depends in principle on ascertaining 

 the sagitta or deflection in the image of a straight line along one side 

 of the plate. In the discussion it is shown that to give the total dis- 

 tortion near the edge of the plate would not answer practical require- 

 ments, and that the proposed method of examination does give the 

 most useful information that can be supplied. 



(14.) Definition. This is found by ascertaining what is the thinnest 

 black line the image of which is just visible when seen against a 

 bright back-ground. It is shown that this is the best method that 

 could be devised of measuring the defining power of an objective, 

 and that it is not open to serious objections on theoretical grounds. 



(15.) Achromatism. Under this heading is recorded the difference 

 of focus between an object when seen in white light and the same 

 when seen in blue or red light. How to estimate approximately the 

 diffusion in the image due to any defect in achromatism is discussed. 



(16.) Astigmatism. This test is performed by measuring the dis- 

 tance between the focal lines at the corner of the plate, and by cal- 

 culating from the result thus obtained the approximate diameter 

 of the disc of diffusion due to astigmatism. The reflection of a lamp 

 in a thermometer bulb is employed to give a fine point of light, and 

 the positions of the focal lines are obtained by noting when the imnge 

 of the object appears as a fine line, first in one direction, and then 

 in another, as the focus is altered. 



B 2 



