The Focometry of Lens- Combinations. 15T 



whilst the most general expressions for a, /3, y are 



It appears, then, that the generalized expression for Gr 

 must be taken to be 



[_dx c J r OycJ r ^z cj r J 



\"d* 3y d*/ .e dt r 



so that the kinetic energy density in the field is expressed in 

 its most general form by 



^(H 2 + G') = ~{(curlA)* + (divA) 2 }. 



X. The Focometry of Lens- Combinations. 

 By Prof. A. Anderson, M.A.* 



IT is probably not possible to improve much on the 

 ordinary nodal-slide method of measuring the focal 

 length of a converging or diverging lens-combination. This 

 consists in allowing beams of light from an object, which 

 are made parallel beams by a convex lens, to pass through 

 the combination, which then, either directly or by the help 

 of a convex lens, forms an image of the object on a screen. 

 A vertical line intersecting the axis of the combination, about 

 which a small rotation of the combination produces no dis- 

 placement of the image, is then found. This vertical line 

 intersects the axis of the combination in H 2 , the second 

 principal point. The image formed on the screen, or, in the 

 case where a convex auxiliary lens is used, the conjugate of 

 this image with respect to the lens, is F 2 , the second principal 

 focus. H 2 F 2 is the focal length. 



* Communicated by the Author, 



