1899J MICROSCOPICAL JOURNAL. 309 



it is with these that we compare the best results of the 

 achromatic with the performance of the apochromatic. 



Space will not allow explaining what is meant by pen- 

 etrating power, which is of course the reciprocal of the 

 resolving power, or how the numerical aperture can be 

 measured by the Abbe apertometer ; neither can room be 

 found to show how definition may be affected by bad cen- 

 tring of the lenses by which is meant the irregularity of 

 the alignment in the optic axes, the parallelism of their 

 planes or in the setting of their planes at right angles to 

 the optic axis; neither can the reader be afforded infor- 

 mation upon the effects produced in the performance of a 

 lens by using a solid cone of light from the condenser or 

 a hollow one ; axial illuhiination or the other form to 

 which the term "oblique light" is usually applied, these 

 remarks are somewhat foreign to the comparison of 

 achromatics with apochromatics, and would more prop- 

 erly be found in a brochure on lens testing, or in a text- 

 book on the microscope. — III. An. Micr. 



Amoeba and Their Differentiation from Body Cells. 



DR. FEINBERG, (FORTSCHRITTE DER MEDICIN XVII., No. 4. 

 It is frequently very difficult, indeed of ten impossible, 

 for even a trained eye to distinguish an animal or human 

 cell from a free amoeba. The author mentions two ways 

 by which the distinction can be made with certainty, mz., 

 by cultivation and staining. He used a solution of com- 

 mon salt of different degrees of concentration, in which 

 one or several organic substances were placed. Upon these 

 the amosba developed within about three days with ex- 

 treme rapidity. After about 8-14 days the substances 

 were overgrown and infiltrated with amoeba, and even the 

 fluid contained numerous protozoa. Such cultures the au- 

 thor was able to preserve for nine mouths, and after this 

 time obtained fresh cultures by re-inoculation upon new 



