WELCKER, ON MICROSCOPIC DIAGNOSIS. 58 



In opposition to this, I would remark : that in transmitted 

 light there are no shadows (Schlagschatten) at all, and in 

 direct illumination only when the light falls upon an opaque 

 object. If an opaque object having vertical elevations on its 

 surface be viewed under the simple microscope by direct light 

 every elevation will exhibit a shadow on the side which is 

 turned from the light. But if a transparent object, that is to 

 say an object which acts as a lens — as for instance, a starch 

 granule lying on a black surface — be viewed with a lens by 

 direct light, the less illuminated side of the granule, but 

 which is obviously not in shade, will be that which looks 

 towards the light, whilst on the opposite side will be exhibited 

 its focus. That this distribution of the bright and dark, due 

 to the lenticular nature of the object, is, it must be confessed, 

 deceptively similar to the light and shade of ordinary illumi- 

 nation, I have already (loc. cit.) remarked. 



In cases where, by transmitted light, a visible shadow is 

 exhibited, this is always consequent upon oblique illumination 

 (whether produced by a lateral position of the mirror or of 

 the diaphragm, or in any other way, is immaterial), and it is 

 only to such cases that Harting's statements above quoted 

 can refer, although in the section in which they occur the 

 expression " oblique illumination" is not employed.* But 

 when Harting says that the shadows of an elevated object, 

 owing to the reversal of the image in the compound micro- 

 scope, fall on the side looking toivards the light— it is obvious 

 enough that that is the side on which a " shadoAv " must be 

 placed. But the phenomenon is not so at all, for we find that 

 the supposed shadows of elevated objects viewed in the com- 

 pound microscope are situated on' the side which looks from 

 the mirror. 



The supposed " shadow" of convex transparent objects, as 

 I again repeat, is not a shade at all, but it is the less illumi- 

 nated portion of a body which acts like a convex lens. In 

 the simple microscope we perceive the focus which is turned 

 to one side by the oblique illumination, on the side looking 

 from the mirror, and in the compound microscope on that 

 which looks towards it. 



II. Diagnosis by movement of the tube. 

 For the distinguishing of elevations and depressions 



* In § 201 it is expressly stated that oblique transmitted light acts in 

 exactly ihe same way as does the oblique position of the object, that is to 

 say, by the production of shadows. 



