ON THE THEORY OF THE MICROSCOPE. 249 



without contravening the very first requirement of corrected 

 spherical aberration. * 



XXII. In connection with the foregoing statements, a few 

 jhints respecting the most suitable modes of testing microscopes 

 ;may be here given. According to past experience, it would be 

 [considered justifiable to estimate the worth of an objective accord- 

 ling to the minuteness of ultimate detail which it might render 

 visible, and, acting upon this view, to consider the resolution of 

 ! difficult test objects as the proof of highest performance. For 

 though it could not be denied that the particular kind of detail in 

 I these test objects, and the particular mode of illumination applied 

 to them are not such as occur in ordinary work, it might, at 

 least, seem beyond doubt that the same peculiarity of construction 

 which gave so good a result in these exceptional cases must be 

 operative in ordinary work. This must, however, now be 

 objected to, for the reasons above given. A mode of testing 

 which turns upon the determination of the utmost limit of 

 <' resolving power," whether tried upon a ''Kobert" plate, a 

 diatom, or an insect scale, brings into play a quite exceptional 

 direction of rays of light into the microscope, such as is, indeed, 

 required for this purpose by the physical condition of the problem, 



* The dry objectives made on Abbe's calculations, founded upon the 

 principles before explained, have only 105 to 110 deg. of angular aperture 

 for the highest ^powers, and cannot pretend, therefore, to compete, in 

 resolving diatoms, &c., with objectives of much higher angle. The 

 immersion lens is constructed with a free aperture of about 100^ in water, 

 i,e., somewhat more than would correspond to 180° in air, because this is 

 attainable without serious disadvantage. Professor Abbe is, however, con- 

 vinced that even the immersion lens would not lose any of its value for 

 ordinary scientific purposes, whilst it would be materially improved in 

 many respects if its construction were based upon calculations for a smaller 

 aperture, " but," he adds, "in view of this universally accepted standard 

 of valuation,' the practical optician can scarcely be expected to trouble 

 himself about qualities of performance which would be very certainly 

 ranked amongst those of a secondary order !" 



