1898.] MICROSCOPICAL JOURNAL. 119 



by the microscopic methods still too current in our 

 schools, which have taken their methods from Germany. 



At the Jena workshop in 1895, Prof. Zimmermann, 

 one of the scientific staff (who has himself published a 

 work on microscopy), said that in photographing they 

 found no difference in results obtained by the chromatic 

 and achromatic condensers ; which is equivalent to the 

 statement that they knew of no better results than those 

 from a 0*50 cone. Our results are quite different. Mr. 

 A. Pringle, whose splendid photographic work on bacteria 

 is well known, often uses the largest aplanatic cones ; 

 and, Dallinger : "Photo-micrography with a small cone 

 is quite easy, as great contrast can be secured [the 

 reason has been shown in foregoing paragraphs]. With 

 a large cone difficulties begin — difficulties of adjustment, 

 difficulties of lens correction, difficulties of exposure, and 

 difficulties of development. If, so far as our experience 

 goes, a good photo-micrograph is required, these difficul- 

 ties must be mastered." 



21. This quotation leads us to the prejudicial effect of 

 the theory (or rather of its undue preponderance) upon 

 microscopic objectives. The mode of illumination 

 directly influences the quality of the objective ; because 

 the all-important point of correction for spherical aberra- 

 tion has commanding influence upon the cone of hetero- 

 geneous rays which can be used with it. This does not 

 appear under the Abbe method ; and Strahl maintains that 

 "the influenceof spherical aberration has been consider- 

 ably over-rated in objectives !" The most eminent firm of 

 Continental opticians states that its lenses, owing to the 

 system of calculation and manufacture, are uniformly free 

 from spherical aberration, so much so that there is no need 

 for any " empirical tests," viz., testing upon the micros- 

 cope itself. That is not the case when tested by the 

 more perfect English appliances. The condenser itself is 

 an English appliance. Ten years ago only one house, 



