tives one receives on its front surface and transmits a 

 larger number of rays than another of equal magnify- 

 ing power, we have a case where power would indicate 

 that we should see equally well, but we will find that 

 there is a difference, due to the amount of angular 

 aperture, in favor of the wider angle. Or, in the case 

 of two objectives in which one has one-half the power 

 of the other, but which transmits the same amount of 

 rays, it would appear that if the power alone were to 

 indicate the visibility of the object, the higher one 

 should show more detail, whereas in reality both show 

 equally well. The practical value of this fact is most 

 apparent near the limit of the resolving power of micro- 

 scope objectives. 



Aperture. Without the defining word ' angular,' 

 aperture indicates a very important feature in an 

 objective and designates the diameter of the beam or 

 pencil of light which passes out through the rear lens 

 of an objective, or in other words, is the effective 

 diameter of the rear lens. 



The word * aperture ' has been used, especially in 

 England, to designate the angular aperture. This has 

 created confusion and if an abbreviation be used the 

 word angle is far more suitable, as Dallinger says in 

 Carpenter's, " The Microscope and its Revelations," 

 " We would, nevertheless, remark that visibility of detail 

 in, for example, the moon depends on the aperture of 



72 



