Angular Aperture Defined. 51 



The first question that arises is, can we, by adopting any 

 definition for angular aperture, express the power of an ob- 

 jective to resolve bands of closely ruled lines ? This, it occurs 

 to me, is the only definite test that we have for an objective. 

 The plain question is, can we adopt any definition for aperture, 

 which will at once tell us the power of a lens to resolve tests ? 

 In other words, can we express, in these terms, just what is 

 the lowest limit of aperture that will resolve the individual 

 bands of Nobert's plate ? If we can do this, by any definition, 

 then by all means let it be adopted at once. If we cannot do 

 this, then let us adopt the simplest one possible. My impres- 

 sion is, that we cannot, in any possible way, express, by any 

 terms of aperture, the capabilities of a lens; and certainly not 

 until some more definite figures are used than the + 180° with 

 which we are now so familiar; for this angle is given to a class 

 of objectives which have apertures ranging from a certain in- 

 determinate point upwards. To my mind, it appears that it 

 will never be possible to express resolving power, under ordi- 

 nary conditions, in terms of aperture. 



The appearance of an object depends upon two factors, viz. : 

 the aperture of the objective (whatever this may mean), and 

 the angle of the illuminating pencil. 



Again, in the present state of our knowledge, we are not 

 able to say how much the angle of field affects our resolutions. 

 I am inclined to the opinion that it has no little influence; 

 but, without having any well-formed opinion on the subject, I 

 point out a single way in which the effect may be seen. 

 When we increase the diameter of our field lens, and keep the 

 angular aperture the same, the result is an increase of working 

 distance, and, as a matter of course, our lens has a greater 

 depth of focus. That this has some effect upon the perform- 

 ance of our lens cannot be questioned. How much, and just 

 what, we cannot tell. 



In selecting a definition we should first settle upon certain 

 requirements, which it must fulfill, and these may be given as 

 follows: 



1. It must be concise, and must admit of no misunderstand- 

 ing. 



2. It must be one which can be practically applied in meas- 

 uring aperture, without dispute as to the accuracy of the 

 results obtained. 



