330 Address by the President. [Sess, 



There are three qualities desired in object-glasses — defining 

 power, resolving power, and penetrating power. The first and 

 second and first and third should always be united, but the 

 second and third are opposed to each other, and cannot be 

 united in the same object-glass, so that the kind of object- 

 glass needed depends upon the object sought. 



Defining Poxver. — This is the power of an object-glass to 

 show clearly and distinctly the contours of an object. There 

 should be no indistinctness about the edges — the image should 

 stand out clear and crisp. This power of good definition 

 depends upon the good correction of the lenses for spherical 

 and chromatic aberration. 



Resolving Power. — This is the power of the object-glass to 

 separate, and thus make visible, the fine structure of the 

 object. For instance, if there are fine lines or dots in the 

 object, resolving power enables these to be seen. This is a 

 power different altogether from that of magnification : an 

 object-glass may magnify 500 times and yet not show fine 

 details so well as one magnifying only 250 times. Magnifica- 

 tion depends upon the length of focus of the object-glass ; 

 resolving power upon its numerical aperture and the number 

 of rays of light from the object it is capable of admitting. 

 This raises the question of the theory of diffraction spectra, 

 which want of space forbids us entering upon. Those who 

 wish to study the subject should consult Professor Abbe's 

 papers in the 'Journal of the Eoyal Microscopical Society.' 



Penetrating Power. — This is the power which an object- 

 glass possesses of catching up and making visible more than 

 one plane or stratum of an object, and is in inverse ratio to 

 the resolving power, and is thus, other things being equal, 

 best in object-glasses of moderate numerical aperture. As has 

 been said, the object sought determines the kind of object- 

 glass required. 



As frequent reference has been made to the image formed 

 by lenses, it may be well to look at the optical principles on 

 which images are formed. Optical images are of two kinds — ■ 

 real and virtual. Eeal images are such as can be received 

 upon a screen — such, for instance, as the image cast upon a 

 screen by the projection lantern. Virtual images, on the con- 



