122 PRACTICAL MICROSCOPY. 



macroscopical, and to become familiar with the idea that it 

 is a thing sui generis, in regard to which nothing can be 

 legitimately inferred from the optical phenomena connected 

 with bodies of large size." 



Professor Abbe discovered the fact that the microscopical 

 image is the result of diffraction, or the consequence of 

 those changes which are produced in rays of light by their 

 interception by minute particles ; the rays are collected at 

 the back of the objective, where they depict the direct and 

 spectral images of the source of light, reaching in their 

 further course the plane which is conjugate to the object, 

 and give rise there to an interference phenomenon, which 

 gives the ultimate image observed by the eye-piece, and, 

 therefore, the image depends essentially on the number and 

 distribution of the refracted beams which enter the objec- 

 tive.* From this it appears that the larger the number of 

 diffracted rays admitted into the objective the greater like- 

 ness to the object will the image possess, a true image being 

 only produced when all the diffracted rays from the object 

 are admitted. 



Dr. G. Blackham, in a paper read before the Microscopical 

 Congress at Indianapolis, August 15, 1878, said, "Now, 

 if it is the function of the objective to collect and bring to 

 a focus rays of light too divergent to be received by 

 the unaided eye . . . the more of these lost rays that 

 a given glass can so collect and bring to a focus the 

 better the glass," and " one would naturally expect to find 

 that the improvement or evolution of the microscope was 

 accompanied by an increase of the angular aperture of the 

 objectives, and this, indeed, we find to be the case." Un- 



* It is clearly beyond the scope of a work such as this to enter fully into the 

 details of this most important question. Those who wish for a more complete 

 dissertation are referred to the April number of the ' Journal of the Royal 

 Microscopical Society' for 1881, where also Professor Abbe's paper "On the 

 Estimation of Aperture " may be found.^ 



