Notes on some Foco metric Ajtyaratiis. By F. J. Cheshire. 519 



azimuth reading and deducting it from the first one, the angle a is 

 obtained, through which the table must be rotated to move the 

 image through a distance of 1 mm. in the focal plane of the lens. 

 Then it follows that, 



- = tan a 

 f 



When a distant object is employed, no appreciable error is 

 introduced by the lateral displacement of the lens being tested, 

 which must generally occur as the table carrying it is rotated. 

 Theoretically, a near object might be employed, if the first nodal 

 point of the lens were adjusted in the axis of rotation of the table. 

 But this is such a difficult adjustment to make, that it renders this 

 modification of the method valueless when compared with the first 

 arid simpler one. 



To obtain the greatest accuracy a telecentric disk, with the 

 length of its slit vertical, should be mounted in the anterior 

 principal focal plane of the lens, and a distant horizontal line 

 should be employed for adjusting the millimetre scale in the posterior 

 principal focal plane. 



