266 THE AMEKICAN MONTHLY [December, 



same size as the image of the mirror in the objective used, and is for 

 the purpose of intercepting this image in the objective. M is the ob- 

 jective and O is the standard sci'ew for body-tube (Fig. 8) . 



It will easily be seen that by moving the stop the image of the mirror 

 can be intercepted at any point from the centre to the extreme edge of 

 the objective. The best effects are obtained when the stop is placed a 

 little beyond the centre, as at C (Fig. 2). 



The motions indicated can all be reversed by means of the revolving 

 fitting of the nose-piece. Changes from dark field to bright, and the 

 reverse, can be instantly made by sliding the carrier in its fittings. 



The effects obtained by the use of this piece of apparatus with hom- 

 ogeneous-immersion lenses are very remarkable. The internal organs 

 of infusoria are shown with a precision and beauty never equalled. 

 Bacteria in fluids are seen as brilliant points of light. Vast numbers 

 that are above and below the focus, and which could not be seen in a 

 bright field, are brought into view. The trachea of mosquito larvae can 

 easily be traced as beautiful thread-like lines throughout their entire 

 length. 



In examining stained human muscle containing trichina, and using a 

 t dry objective with bright field, great care is required to see the para- 

 site in its cyst, but on a dark field produced by the use of the stop the 

 worm will be seen as a brilliant coil and can be plainly traced from tip 

 to tip. 



Carmine Staining for Nervous Tissues. 



By GEORGE A. PIERSOL, M. D., 



PHILADELPHIA. 



Present methods of imbedding and sectioning render staining en 

 masse desirable in all cases where practicable, to which end borax- 

 carmine, alum-carmine, and Delafield's haematoxylin are our most 

 reliable and valuable reagents ; of these, borax-carmine is undoubtedly 

 the most widely employed, and, owing to its great power of penetra- 

 tion, certainty, stability, and uniformity of action, this preference is 

 well deserved. Notwithstanding the admirable differentiation and 

 crisp pictures obtainable by properly used borax-carmine, which for 

 many cases leave little to be desired, the exhibition of the ganglion 

 cells of the central nervous system is usually far from satisfactory, lit- 

 tle more than the nucleus, or at best the bodies of the cells, being well 

 brought out ; a conspicuous example of this short-coming of the ordi- 

 nary carmine or haematoxylin stains is seen in the large ganglion cells of 

 the cerebellum. Any method of offering a ready means of securing 

 the satisfactory display of these elements, while staining en masse, 

 must be of interest to the histologist. Now more than a year ago, my 

 friend, Dr. O. Schultze, of Wurzburg, Germany, called my attention to 

 the results of some experiments which he had made with sodium car- 

 minate ; the beauty with which the nerve cells of all parts of the central 

 nervous system were shown at once aroused my interest in the method, 

 which during the past year I have further tested in the Histological 

 Laboratory of the University of Pennsylvania with veiy gratifying 

 results. 



