412 Major L. Darwin. On the Method of 



ing bar is moved from side to side. The ground glass can be brought 

 approximately into focus by means of the already-mentioned move- 

 ment to and fro on the swinging beam, but for accurate adjustment a- 

 slow motion arrangement is attached to the movable part itself ; 

 the handle H gives the required motion, and there is a scale S, called 

 the " focus scale," by means of which these small movements can be 

 accurately measured. On the lens-holder there is a movement, corre- 

 sponding to the swingback of an ordinary camera, by which the lens 

 can be made to revolve vertically round a horizontal axis, without, of 

 course, any corresponding movement of the ground glass ; there is a 

 vertical arc, V, by means of which we can read off the vertical angles 

 through which the lens is rotated. An arrangement is also suppled 

 by means of which the lens can be moved backwards and forwards on 

 the movable stand, thus allowing the position of the lens to be so 

 adjusted that the horizontal axis can be made to pass through any 

 point in its axis. 



9. Effective Aperture of Stops. 



The effective aperture of one or more of the various stops supplied 

 with the lens is found by a well-known method. The image of a, 

 very distant object is first brought into focus on the ground glass 

 of the testing camera; a collimator, which has itself been pre- 

 viously focussed on a distant object, may be used instead of the 

 distant object ; the ground glass is then taken out and exactly re- 

 placed by a tin plate with a small hole at the centre; this hole, 

 which should be very small, will, therefore, be at the principal 

 focus of the lens. The room being darkened, a gas burner is 

 placed behind the small hole, and thus parallel rays, in the form 

 of a cylinder, are made to issue from the lens towards the front. 

 A piece of ground glass, with a graduated scale engraved on it, is 

 now held in front of the lens, and the diameter of the illuminated 

 Use, or section of the cylinder as seen on the glass, is directly 

 measured off as any stop is inserted in its place. Thus is found the 



