JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. Ill DECEMBER 4, 1913 No. 20 



PHYSICS. — Determination of the zonal variation of the equiva- 

 lent focus. E. D. TiLLYER. Bureau of Standards. Com- 

 municated by Fred. E. Wright. 



The method of lens testing developed by Hartmann^ in 1904 

 has given a very efficient means of determining the grouping of 

 the Hght rays after passage thru a lens system. This method is 

 laborious and difficult when applied to the zonal variations of the 

 equivalent focal length. In the present paper it is proposed to 

 show how one set of measures gives both the variations of the 

 position of the focus and of the equivalent focal length: also, by 

 making several sets in light of different wave lengths, the axial 

 and oblique chromatic aberrations can be obtained. 



A diagramatical sketch of the apparatus used, is given in figure 

 1. A nearly parallel beam of light from a distant monochromatic 

 source falls upon a metal plate P pierced with small holes, only 

 two of which are shown, forming hght rays which pass thru the 

 lens system and are refracted in the normal manner. A shadow 

 photograph is taken upon a plate placed close to the lens, a second 

 at a considerable distance on the other side of the focus. These 

 shadow images are not as sharply defined as when both exposures 

 are made close to the focus. If, however, care is taken, a dark 

 center diffraction disk can be obtained which increases the ease 

 and accuracy of a setting. 



Now consider the position of any ray R (fig. 1) with reference 

 to the axial ray. Let a, b equal the distances of this ray from the 



1 Hartmann, Zeitschr. f . Instrumentenkunde. 1904. 



481 



