54 WELCKEK, ON MICROSCOPIC DIAGNOSIS. 



Harting (§ 885) further recommends the "changing of the 

 distance of the object," which in most cases suffices, unless 

 the elevation or depression be altogether inconsiderable. 

 But it is precisely in the latter case, in which according to 

 Harting the movement of the tube leaves us in the lurch, 

 often that the determination appears to be most important 

 and desirable. In this case, it may be necessary, according 

 to the same author, " to view the object in a direction per- 

 pendicular to that in which it was first placed ;" and he cites 

 the acetabular depression of the blood-corpuscles, and the 

 hollows on lignified vegetable cells, as instances in which 

 this kind of profile view affords the wished-for result. But 

 how often is it not altogether impossible so to place certain 

 small objects in such a position, or to procure such a section 

 of them, as would exhibit the elevation or depression in 

 question, in profile ? In these cases, however, I would 

 suggest as a means of diagnosis the elevation and depression 

 of the tube, inasmuch as I have proved that the most minute 

 punctiform elevations and depressions, ivhilst acting as convex 

 and concave lenses, exhibit different optical characters. The 

 course of the rays of light through spherical objects, some of 

 which act like convex and others as dispersive lenses, is ably 

 discussed by Hartmg (§ 275) and illustrated by figures ; but 

 I do not find that he has either confirmed or contradicted 

 my statement, that all extremely minute convex and de- 

 pressed objects, as representatives of which I adduced the dry 

 spermatozoid and the lines on a micrometer, appear black 

 or light according to the position of the tube (the convex 

 appearing light on the elevation and the depressed on the 

 depression of the tube), and that this phenomenon is refer- 

 able to the lenticular property of the objects, and affords a 

 certain, frequently the only means by which to judge of 

 their figure. 



This is not the place again to point out the various pre- 

 cautions to be observed in the employment of this method, 

 for which I must refer to my former communications. 



In conclusion, 1 give a tabular summary of the various 

 possible modes in which the light and dark may be disposed 

 in objects illuminated obliquely. 



