Notes on some Focometric Apparatus. By F. J. Cheshire. 517 



example, in a particular case, 1 mm. of the collimator scale project 

 on to 16 divisions of the eye-piece scale, then the focal length of 

 the lens being tested is 16 mm. 



When the telecentric disk is placed in the plane of the coincident 

 principal foci of the collimator lens and the lens being tested, the 

 system formed by these two lenses is a telecentric one, both on 

 the object and the image side, so that no error is introduced if the 

 scale h is not correctly adjusted in the principal focal plane of the 

 collimator lens. As has already been pointed out, this system is 

 also independent of any focusing error of the auxiliary Microscope. 

 This arrangement, therefore, gives the best possible results, although 

 as a matter of fact results of quite sufficient accuracy for most of 

 the purposes for which focal lengths are required, can be obtained 

 by supporting the lens being tested on the telecentric disk itself. 

 With eye-pieces it is desirable to employ a yellow-green colour- 



Microscope 



Fig. 55. — Diagram of optical-bench focometer system. 



screen to limit the light, more or less, to the most visible part of 

 the spectrum. The focal lengths of negative lenses, provided they 

 are not greater than the working distance of the objective of the 

 auxiliary Microscope, can be determined with equal facility. 



Optical-bench Focometer. — In a paper, read before this Society 

 in 1884, entitled " Note on the Proper Definition of the Ampli- 

 fying Power of a Lens or Lens System," it was laid down by 

 Professor Abbe that " the reciprocal of the focal length of an 

 [optical] system is by itself the proper definition of its amplifying 

 power, because this reciprocal expresses numerically the visual 

 angle (measured by its tangent) under which the unit of length 

 appears through the system." Curiously enough the French 

 physicist Verdet* had anticipated Abbe by some twelve years in 

 the formulation of this definition. 



See CEuvres de Verdet, iv. (Paris 1872) p. 944. 



