

Examination of Photographic Lenses at Keiv. 431 



case is more difficult. Probably the most important consideration is 

 that the test should not be based on a mere judgment, the reason for 

 which one person cannot readily communicate to another. In many 

 works on photography the extent of field over which the lens pro- 

 duces a " sharp " image is discussed, as if by mere inspection this 

 could be determined ; whereas no two people would exactly agree as 

 to where the diffusion of the image was sufficient to be classed as 

 want of sharpness, and no two objects would serve equally well for 

 such a test. It is essential, at such an establishment as the Kew 

 Observatory, that the observer should obtain some definite numerical 

 result from his examination, even though it may be considered 

 advisable to merely employ general expressions in the wording of the 

 certificate ; under any other system it would be impossible for any 

 length of time to prevent the standards from varying. 



Still more difficult is it to avoid errors from actual variations in 

 eyesight, whether between different individuals or at different times 

 in the same individual. Some general conditions may, however, be 

 laid down. When the illumination of an object is very feeble, the 

 subjective light of the eye, as it has been called by Helmholtz, plays 

 an important part in determining the least intensity of illumination 

 which is visible, and this subjective light is a very variable quantity ; 

 the eye increases in sensitiveness for a long time when light is 

 excluded from it, the increase at first being very rapid, which may be 

 another way of expressing the same fact. Hence, any feebly illu- 

 minated object must be a bad test-object, for its appearance will vary 

 very materially according to the state of the ^eye. On the other 

 hand, if the illumination is too bright, the eye will be much in- 

 fluenced by irradiation, and the subjective effect on the eye will be a 

 bad indication of the true condition of the object; moreover, as 

 irradiation is the effect on the appearance of an object produced by 

 brighter surrounding objects, and as this effect diminishes as the 

 differences of shade get less, the test-object should show no marked 

 contrasts in illumination. But in applying these general remarks to 

 the case under consideration, it must be remembered that it is not 

 the test-object which is seen by the eye ; it is the image of the test- 

 object, as produced by the lens under examination. Hence, it 

 appears that the test-object should produce an image of medium 

 intensity of illumination, and one in which there are no great differ- 

 ences in shade. The test-object used at Kew, it will be remembered, 

 consists of a perfectly black object seen against a bright background, 

 and it might therefore appear as if it were not a good selection. In 

 order to prove that, as a rule, the differences of shade in the image 

 are small, and that no objections can be raised to the Kew test on 

 theoretical grounds, it is necessary to show what is the effect on the 

 image produced by a want of defining power in the lens. 



