ON THE THEORY OP THE MICROSCOPE. 215 



pound microscope of the present day is constructed ; and on this 

 basis, as will be shewn in the sequel, many questions bearing 

 with equal importance on the theory of the microscope and on its 

 rational construction, find their solution — such, for instance, as 

 respect the exact seat of various sources of error, the means of 

 correcting them, the limits of perfection which may be possibly 

 assigned under given conditions, and the influence separately 

 exerted on the quality and sum of the total effect by the several 

 factors; focal length of objective, length of tube, and strength of 

 ocular. 



VII. In the foregoing remarks the chief points have been 

 indicated from which an exhaustive theory of the microscope 

 must be set forth, Prom ^them may be gathered a theory of 

 aberrations, or faulty formation of images, sound and strong 

 enough to master the difficulties which the application of excep- 

 tionally large angles of aperture to microscope objectives has 

 occasioned. 



It appears that these faults of image formation are separable 

 into two distinct classes, one comprising faults of the focussing 

 act (aberrations in the strictest sense), the other comprising faults 

 of the amplifying function (extension of image over a larger 

 superficies), which latter have not hitherto been considered. To 

 the first class belong those spherical and chromatic aberrations 

 commonly studied ; in the second class must be placed a series of 

 peculiar deviations of rays of light from their normal course, 

 which arise from the circumstance that the separate rays 

 of a homofocal beam occupying the aperture of the lens 

 yield unequally magnified images, according as their inclination 

 to the axis varies, and according also to the unequal refran- 

 gibility of the different colours — an inequality wbich obtains just 

 as much whether the several partial images are compared with 

 each other, or whether within the area of each image different 

 positions in the field of vision are compared. Prom these 

 deviations, which Professor Abbe terms anomalies of amplification 

 rather than aberrations ^ arises that well-known indistinctness of 



