78 THE AMERICAN MONTHLY [March, 



glass it might ahiiost be inappreciable. When the elements of a minute 

 structure are reduced to only a few wave-lengths apart, the whole dif- 

 fraction fan is spread out over more than iSo° in glass, so that a medium 

 of a higher refractive index must be used to condense them within the 

 limits of any objective. 



The full force of the Abbe theory, however, is only realized when it is 

 considered that not only must the objective be of the largest angle to 

 take in these widely diverging ditiraction spectra, but that with them 

 must also be combined at the same time the dioptric image formed by the 

 central rays. " The images which are obtained with oblique light will 

 always be incomplete, and not similar to a geometrical projection of the 

 object, and generally (though not always) more dissimilar than those 

 by central light in regard to the minuter details. Strictly similar im- 

 ages can not be expected, except with a central illumination with a 

 narrow incident pencil, because this is the necessary condition for the 

 possible admission of the whole of the diffracted light." This requires 

 that the objective shall be so accurately corrected that the chromatic 

 and spherli^al aberrations shall be completely eliminated, so that the 

 central and peripheral rays can be utilized without change of focus. 

 How few even of the best objectives fulfil this requirement every 

 worker knows only too well. Here any superiority of the Zeiss apo- 

 chromatics will be shown. It must, however, be remembered, first, that 

 the difference in degree between the performance of the best American 

 and English achromatic lenses and the Zeiss apochromatics is often 

 only detectable by the experienced eye ; and, second, that the vast bulk of 

 microscopical work is done with medium or low' powers, where the dif- 

 fracted beams play a much less important part proportionately, the 

 image being chiefly dioptric. As to the first point, the writer may 

 even go further and say that it is a contested point among many of the 

 highest authorities in America whether the Zeiss apochromatics have 

 any marked practical superiority. The student may rest contented, 

 therefore, that he can work for an indefinite time with any good Amer- 

 ican or English lenses w^ithout losing his time or running any serious 

 risk of making mistakes in what he sees. This undue praise of Abbe 

 and Zeiss is a serious fault in so able and catholic a writer as Dr. Dal- 

 linger, and the repeated allusions to them suggests there is some per- 

 sonal debt to be paid, and the distinguished editor seems to be somewhat 

 over anxious to cancel his obligations. On the the other hand, his 

 condemnation of the cramped, awkward, continental form of stand will 

 be appreciated, and it is to be hoped will check the present craze— for 

 it is little else — for that form of instrument. 



There are many other topics wdiich ought to be treated, but there is 

 not room in the pi'esent article to more than allude to them. The 

 criticism of the swinging sub-stage is too wide and sweeping, the 

 necessity of high-angled achromatic condensers for all work, so emphat- 

 ically urged, is a doubtful question, the condemnation of the Ross-Zent- 

 mayer arm and its accompanying form of fine adjustment seems too 

 broad, and the recognition of American work and workers quite too 

 scanty to be fair. Strange to say, the subject of photo-micrography 

 has been entirely neglected. 



The portion devoted to practical methods is very good, though so 

 brief that the student will still need the help of such aids as Martin, 



