Major L. Darwin. On the Method of 



11. Principal focal length = - - ins. 



Back focus, or length from the principal focus to the nearest point 

 on the surface of the lenses = ins. 



The following is the method of finding the principal focal length 

 with the testing camera. By means of the mark (see fig. 1), on the 

 three-legged stool, the swinging beam can be brought approximately 

 to a central position; there are also two iron stops, T and T', remov- 

 able when not wanted, which, when in position, prevent the swinging 

 beam from passing beyond these points. These stops (or, more accu- 

 rately, the iron plates on the swinging beam with which they come 

 in contact) are capable of adjustment, and thus a means is obtained of 

 allowing the beam to be revolved about A as a centre, through a 

 known angle, with great ease and accuracy. After the focus has 

 been very carefully adjusted for a distant object, and after the beam 

 has been brought approximately to the central position by means of 

 the mark on the stool, the image either of some well-defined object 

 seen through a hole in the window-shutters, or of a mark in the 

 collimating telescope, is made to appear on the centre of the engraved 

 line on the ground glass ; this can be done by raising or lowering one 

 or more of the legs of the stool or by moving it laterally; this 

 adjustment being accurately made, the line joining F, the centre 

 of the ground glass, and the centre of the lens, if prolonged, 

 will pass through the distant mark; when once made, this adjust- 

 ment will hold good, with sufficient accuracy, for all lenses which 

 may subsequently be placed in the testing camera. Now, when the 

 swinging beam is moved from side to side, the image appears to run 

 along the engraved line on the ground glass ; the position of the 

 image is first noted when the beam is in contact with the stop T, and 

 afterwards when in contact with the stop T' ; twice the distance, as 

 measured on the scale, between these two points gives the principal 

 focal length of the lens under examination. 



In order to ensure accuracy, certain precautions must be taken. 

 The object must be so far off that the distance between its focus and 

 the focus of a point in the same direction at an infinite distance is 

 considerably less than the probable error of observation. The chief 

 difficulty of finding the principal focal length, in the Kew method, 

 and, indeed, in all methods, consists in obtaining an accurate adjust- 

 ment for focus ; and since, for a given error in focus, the greater the 

 aperture the more diffusion there is in the image, the largest stop 

 should always be used when focussing ; but there is no objection to 

 slipping in a smaller stop after the focus is taken so as to obtain as 

 sharp an image as possible, and thus make it easier to read the posi- 

 tion on the scale with accuracy. 



Before proving that the result above obtained is, in fact, the prin- 



