Examination of Photographic Lenses at Kew. 435 



would say, the brilliancy of small objects. In fact, since the definition 

 of an objective could only be rigorously expressed by a curve (or, 

 more accurately, a surface) with dimensions, it is impossible for any 

 one result to give all the information on this head which might be 

 desirable. 



As the eye is capable of detecting a difference of shade of about 

 1 per cent, of a moderately illuminated field, it will be only necessary 

 for the curve shown in fig. 12 to be tangential to the vertical for 

 1 per cent, of its height to render the image of an infinitely thin line 

 visible in so far as that visibility depends on difference of shade. 

 But take the case of a line not absolutely black, and seen against a 

 bright background; then, in fig. 10, the illumination of the centre of 

 the image will be represented by gc, plus some proportional part of 

 gd- } in comparison with the case of the absolutely black line, it can 

 be shown that the curve must be tangential to the vertical for a 

 proportionately greater distance before the shade of the centre of the 

 image of the infinitely thin darker line will be sufficiently deep to 

 form a visible contrast. For instance, if the line is illuminated to 

 nine-tenths of the intensity of illumination of the field, the curve 

 must be tangential to one-tenth of do (see fig. 10) before this con- 

 dition of things occurs. A test depending on the thickness of a line 

 which is darkened to a definite proportional intensity of the field 

 would therefore present this disadvantage, that there would be fewer 

 occasions on which different degrees of imperfection of definition of 

 lenses would show the same result in testing ; such a test may there- 

 fore in future be adopted at Kew. 



It should, however, be remarked that in the whole of the above 

 reasoning it has been assumed that the minimum proportional diffe- 

 rence of shade visible is the same in a thin line as in a thick one, 

 which can hardly be the case. But this false assumption will not, it 

 is thought, vitiate the general conclusions arrived at. 



It is of course conceivable that the actinic rays will be brought to 

 either a better or to a worse focus than the visible rays ; it is believed, 

 however, that no serious error is likely to result from the test being 

 done by the eye and not by photographic methods ; it is almost 

 certain that the curve representing the edge of a surface will have 

 the same general character in the two cases, and therefore that the 

 results obtained with the eye will be a good indication of those which 

 would be obtained by photography. 



