THE SCIENCE OF MICROSCOPY. 3 13 



science from that physico-mathematical research which should 

 have been its especial work, the practical optician would not hear 

 of defeat. And to his untiring effort and inventive skill, crowned 

 after many trials and difficulties overcome with well merited success, 

 we are indebted for a result which has confuted the fears and 

 predictions of those who abandoned a great project, to be accom- 

 plished by more resolute and hopeful workers. 



The microscope of to-day exhibits the labor and triumph of the 

 constructor. But the theory of its action has scarcely kept pace 

 with the achievement of its construction. Not that there has been 

 wanting, at any time, a general explanation of the mode in which 

 an amplified image is produced in the microscope ; nor that there 

 has been any insuperable difficulty in determining the special 

 characteristics of the focussing function of a set of combined lenses ; 

 nor, again, that the aberrations arising from optical peculiarities of 

 the material used have defied calculation 3 or that the means avail- 

 able for correction and compensation of such aberrations have 

 proved as ineffective as was formerly feared. But rather because 

 practical microscopy has concerned itself very little with theoretical 

 considerations, and has derived but little advantage from such 

 applications as have yet been attempted. For the urgency of a 

 complete theory of the microscope, based upon strict physico- 

 mathematical analysis of the optical problems introduced with the 

 successive increase of angular aperture, was never felt until the 

 construction of wide angled objectives necessitated a closer study 

 of the phenomena attending the process of image formation with 

 pencils of light having a considerable inclination to the axis. Nor 

 until the phenomena of diffraction, conditioned by the small 

 aperture of high power lenses, received that attention which their 

 enormous significance for vision through the microscope demands. 

 Nor, lastly, until the consequences of diffraction, occasioned in the 

 ohject by its own physical constitution, were investigated in 

 connection with the performance of the objective. It is only 

 within a quite recent date that a complete theory could be presented. 



