202 On the Construction of the 



Those microscopists engaged in the special study of the Diatomaceae, 

 and in the demonstration of those fine lines, elevations, and depres- 

 sions found on their surfaces, have, for some time past, known the 

 importance of a correct illumination, and accordingly always directed 

 their attention to the construction or improvement of suitable ac- 

 cessory instruments for this purpose. In the examination of animal 

 tissues, however, no particular attention, as far as I could ascertain, 

 seems to have thus far been directed to the illumination ; at least, 

 I do not remember to have ever seen it made a subject of special 

 remarks in those works on histology which have come under my 

 notice. And still, the difi'erence between the appearance of an 

 object illuminated by central light, and that of the same illuminated 

 by oblique light, is very considerable. 



Knowing then from experience, that the success in resolving 

 those fine hues of many Diatomaceae depended, to a certain degree, 

 upon the correct illumination of the object with oblique hght — 

 provided the angle of aperture of the objective is sufficiently large 

 to admit such a portion of the obhque rays as is required for the 

 purpose — I have employed it more or less for a number of years in 

 my histological studies, and have likewise gained many advantages 

 by it. If we examine, for instance, a number of fine bundles of 

 fibrils, which, containing numerous nuclei or even vessels, cross each 

 other above and below irregularly, we shall find it difiicult to recog- 

 nize the mutual relationships of these elements with the assistance 

 of a central illumination. As all are transparent, and lie one above 

 the other, one part of them is seen through the other, and the whole 

 often presents the appearance of a complicated mass of outlines. 

 The object appears quite difi'erently when illuminated with oblique 

 Hght by means of an achromatic prism.* It now resembles a 

 drawing in bas-relief for by means of the shades produced by the 

 obhque illumination, and reheving the high lights, the true form of 

 the elements of which the tissue consists is rendered more distinct 

 to the eye, which in consequence is better enabled to recognize their 

 mutual relations. The binocular microscope would prove to be still 

 more advantageous for the examination of animal tissues than the 

 prism, that is, if its construction should ever attain such a degree of 

 perfection as to make it adaptable to a combination with objectives 

 of high amphtication. 



In reo-ard to the preparation and preservation of the material 

 used ui these researches, I particularly made use of a weak solution 

 of chromic acid, but also, besides other reagents, of the chloride of 

 trold, as recommended by Cohnheim. The latter proved to be 

 especially useful in the examination of the termination of the nerves 

 on the blood-vessels ; but only in so far as, by its decomposition, the 

 tine nerve fibres are coloured more or less purple, or even brown, in 

 * The prism which I use is that of Abraham. 



