

Examination of Photographic Lenses at Kew. 415 



axis perpendicular to its own axis, it will be seen that, as a rule, 

 something between these two extremes occurs ; commencing from 

 a position when we are looking directly along the axis, no other 

 result than foreshortening the opening is at first produced by the 

 revolution of the lens ; then comes an angle at which the aperture 

 in the stop begins to be eclipsed, either by the mounting of the 

 lenses, or by fixed diaphragms, &c. ; lastly, we come to an angle 

 at which the lozenge-shaped opening appears to vanish, and no light 

 is seen to come from the lens. It is obvious that the intensity of 

 illumination of different parts of the photographic plate varies with 

 the size of the aperture visible from each point ; and, neglecting other 

 considerations for the present, there is thus an inner cone, forming a 

 disc where it cuts the plate, in which the illumination decreases 

 regularly from the centre outwards according to a known law : and 

 there is an outer cone, forming an annulus between where it and the 

 inner cone cut the plate, in which the illumination decreases more 

 rapidly than according to the above-mentioned law; very rapidly, 

 therefore, probably irregularly, on account of the aperture of the 

 stop being successively eclipsed by different parts of the mounting, 

 and certainly according to no law that can be readily stated or ascer- 

 tained. The test now under consideration gives the angles of these 

 two cones. 



The outer cone, which we have called the " cone of illumination," 

 gives the extreme angle of the field of the lens without regard to 

 definition, and is what is known to French authors as the champ de 

 visiUlite, To find the angle of the cone of illumination, the lens is 

 placed in the testing camera, and the observer looks through the 

 small hole in a sheet of tin plate with which the ground glass has been 

 replaced, as in the last test ; the lens-holder is made to revolve about 

 its horizontal axis, and as the axis of the lens moves away from zero, 

 first in one direction and then in the other, the positions at which all 

 light appears to be cut off are noted ; the angle between these two 

 positions as read on the vertical arc, Y, gives the angle of the cone 

 of illumination. 



In order to ensure correct results it is necessary that the axis of 

 rotation should pass through the nodal point of emergence. If in fig. 3 

 AN 2 N\a and BN 2 Ni& represent the extreme rays forming the cone, NZ 

 and NI being the nodal points, it is evident that in order to measure 

 the angle fcNia of the cone the lens must be revolved about N"i, the 

 nodal point of emergence, as a centre. The necessary adjustment is 

 made in the following manner : The image of a distant object having 

 been thrown on the ground glass, the lens is turned through a small 

 angle about the horizontal axis, the glass remaining stationary. If 

 the movement of the lens gives rise to any movement in the image, 

 then the axis does not pass through the nodal point of emergence 



