Examination of Photographic Lenses at Kew. 441 



dominant rays for white light, the definition obtained in the photo- 

 graph itself cannot be perfect. In fig. 19, let u'wu be the position of 



FIG. 19. 



the photographic plate, the focus of which has just been adjusted in 

 daylight ; if the lens has not been properly corrected for achromatism, 

 the different coloured rays will form different cones, and those 

 coming to a focus at w will have a wave-length of about 5570, for 

 that, I am informed by Captain Abney, is generally speaking the 

 dominant wave-length for white light. Let b'bb 1 be the cone of rays 

 of 4420 wave-length, which is not far from the position of the 

 maximum actinic effect for ordinary bromide dry plates, and let uuu 

 be the cone for rays of 4000 wave-length ; since the actinic effect 

 with silver salts begins to fall off rapidly at about 4000 wave-length, 

 the cones outside the cone u'uu may be neglected, and it may be 

 taken that the image of a point covers a disc on the photographic 

 plate of which u'w is the radius. It is evident that what the 

 photographer wants to know, with regard to the achromatism of his 

 lens, is the amount of diffusion caused in the image by any errors in 

 its construction, that is to say, what is the actual size of the disc of 

 diameter u'w. 



The examination for achromatism is therefore made in the follow- 

 ing manner : First the focus is carefully adjusted in daylight on a 

 suitable object placed as far away as possible in the room, and then 

 the focus scale is read off. After this, a sheet of blue glass, the 

 colour of which has a dominant wave-length of 4420, is placed 

 behind the object and close in front of a small opening in the shutter 

 through which all the light enters the room ; the focus is re-adjusted, 

 the focus scale read off again, and the difference in reading to that 

 observed in white light is noted; the length ~bw in fig. 19 is thus 

 obtained. Now let / be the principal focal length of the lens ; and 

 let f be the focal distance when the observation was made, which 

 can be easily obtained with sufficient accuracy by a direct measurement 



