450 Ma jor L. Darwin. On the Method of 



In deciding on the quality of a lens as regards the illumination 

 of the field, this test should be considered in connexion with test 

 No. 10, under which heading are given the angles of the cones of 

 illumination. With regard to the normal use of any lens, except 

 perhaps such as are specially designed for portraiture, certainly the 

 whole of the smallest stop, and, as a rule, the whole of the largest 

 normal stop, should be visible from the whole of the plate ; for if the 

 plate extends much beyond the limits of the inner cone (outside 

 which the aperture begins to be eclipsed) the falling off of density 

 near the edges of the plate will be a serious defect in the photograph. 

 When considering the part of the field lying within this inner cone, 

 it is to be noted that, the wider the angle which the lens covers, the 

 greater is the inconvenience caused by the diminished density near 

 the margin ; if the stop is in front of or behind all the lenses, the 

 intensity of illumination of different parts of the plate can be shown 

 in this case to vary approximately as the fourth power of the cosine 

 of the angular distance from the axis of the lens, and in cases where 

 the stop is between two lenses, the limits of variation will be the 

 third and fourth powers of the cosine of the angle. The following 

 table is therefore inserted to give an approximate idea of the 

 decrease of illumination as we recede from the axis of the objective, 

 the truth lying theoretically somewhere between the two limits here 

 given : 



0. Cos 3 4>. Cos 4 0. 



1-00 1-00 



5 0-99 0-98 



10 0-96 0-94 



15 0-90 0-87 



20 0-83 0-78 



25 0-74 0-67 



30 0-65 0-56 



35 0-55 0-45 



40 0-45 0-35 



45 0-35 0-25 



Eminent lens makers have spoken of the illumination produced by 



their lenses as being uniform from the centre to the margin, but our 



experience is that the decrease is even more rapid than here indicated. 



The above table shows how very objectionable is the use of wide 



angle lenses, whenever they can possibly be avoided. It shows, 



oreover, that the theoretical exposure for different stops should be 



materially modified according to the angle which the lens covers ; 



stance, taking the last column to represent the truth, it would 



right, even though the stops in the two cases had the same C.I. 



umber, to give haJf as much exposure again with a 90 objective as 





