Examination of Photographic Lenses at Kew. 449 



distance from the nodal point, and also (5) with the obliquity with 

 which the rays strike the plate. As far as these two latter considerations 

 alone are concerned, it is evident, therefore, that the illumination, on 

 the plate varies as the third power of the angle of incidence, and also 

 that by multiplying the result obtained on the screen at H by cos 3 

 we obtain the required result on the plate at A. Thus the record in 

 the certificate includes all these first five causes of irregularity of 

 illumination. But there are other causes which are not correctly 

 represented in this method of examination. In lenses not free from 

 distortion the nodal point of emergence varies in position with the 

 angle of incidence, and as the pivot N does not shift its position 

 with reference to the objective during the observation, the condition 

 of illumination of the photographic plate cannot be accurately repre- 

 sented. This is probably a trifling cause of inaccuracy; but one 

 somewhat serious source of error remains to be mentioned. The 

 method of examination does allow for (6) the variation of illumina- 

 tion due to the different amount of glass through which the oblique 

 pencils have to travel ; but, as the observation is made by eye, no 

 allowance can be made for the fact (7) that the actinic rays may be 

 affected in this manner out of all proportion to the apparent varia- 

 tions produced in the visible rays. 



The method of examination adopted at Kew assumes that the 

 light transmitted through the paper, as well as that reflected from 

 the paper, varies in amount with the intensity of the incident light. 

 Captain Abney informs me that his experiments prove this to be the 

 case. But in making the observation the eye should be placed in 

 the same position during both readings ; for we have no reason to 

 suppose the transmitted and reflected lights vary in the same way 

 with the angle of vision. 



It is impossible to suppose that the screen F will be illuminated 

 with perfect regularity, even near its centre, and this must be a 

 source of error, though probably a negligible one. When the axis 

 of the lens passes through H the rays which are brought to a focus 

 at that point, will be parallel to each other as they enter the lens ; 

 but when the axis of the lens is inclined this cannot be the case, for 

 H will no longer be on the principal focal surface ; the screen F 

 should therefore be brought as near the lens as possible, as by that 

 means the part of the screen from which the light comes will be 

 more nearly identical in the two cases. The lamp L should, moreover, 

 be placed as far from the screen as practicable, so as to make the 

 illumination as uniform as possible. With lenses in which the nodal 

 points are some distance apart, the part of the screen from which the 

 light comes will vary considerably with the inclination of the axis, 

 and considerable errors might be introduced by uneven illumination 

 of the screen. 



