I 



Examination of Photographic Lenses at Kew. 417 



the circles which these cones make by cutting the photographic plate, 

 when the focus is adjusted for distant objects, are given in the Certifi- 

 cate of Examination. Having found the principal focal length in the 

 manner to be described immediately, the size of these circles can 

 readily be ascertained by a simple graphical method, which is hardly 

 worth describing in detail. 



In connexion with this test it may be convenient to adopt the use 

 of the term angle of field under examination, (denoted in this paper by 

 20,) to signify the angle subtended at the nodal point of emergence 

 by a diagonal of the plate, or the greatest angular distance which 

 could be included in the photograph, supposing the focus to be taken 

 on a distant object. This angle is found by the graphical method 

 mentioned above for determining the diameter of the circles on the 

 plate, and the result is entered on the Certificate of Examination. 



If the illumination of the field is not to fall off rapidly towards the 

 edges of the plate, for the normal use of the lens we should employ 

 a stop which covers (or nearly covers) the plate of the given size 

 with its inner cone ; that is to say, we should use a stop not larger 

 than the largest stop the whole of the opening in which can be seen 

 from the whole of the plate. In order to find the largest stop which 

 fulfils the above conditions, the lens is revolved about the horizontal 

 axis until the vertical arc reads half the angle of field under exami- 

 nation, and then the different stops are put in one by one until the 

 largest one is found which is seen not to be eclipsed when the observ- 

 ation is made through the hole in the tin plate. The number of this 

 stop is recorded in the certificate. 



The readings taken when measuring the angles of these cones are 

 also utilised for the purpose of adjusting the position of the lens in a 

 manner necessary to ensure accuracy in several of the following tests. 

 The vertical arc is so arranged that it reads zero when the axis of 

 the lens is horizontal, that is to say, when the axis passes through the 

 small hole in the tin plate from which the observation is made ; 

 hence the two readings on the arc when the lens is revolved about 

 the vertical axis, first one way and then the other, so as just to cut 

 off all the transmitted light, should be exactly the same ; if they are 

 not identical the lens-holder is placed in such a position that the 

 reading on the vertical arc is equal to half the difference between 

 them ; then it is evident that the mechanical axis of the objective 

 passes through the small hole, or at all events, cuts the tin plate on 

 the same level as the hole. Now this small hole in the tin plate is 

 in the same position as the centre of the engraved line when the 

 ground glass is in position. Hence, this adjustment being made, in 

 f uture tests we may consider that the mechanical axis of the lens cuts 

 the line on the ground glass near its centre. 



