1 32 Dr. Hi)dgkin and J. J. Lister's Microscopical ObsciTaliutu: 



rounded, and being the thickest part, occasion a depression in 

 the middle, which exists on both surfaces. This form per- 

 fectl\- agrees with the accurate observation of Dr. Young, that 

 on the disks of the particles there is an annular shade, which 

 is darkest on that side of the centre on which the margin is 

 brightest. Though the Doctor drew the obvious conclusion 

 that the disks were concave, he does not consider the fact as 

 demonstrated; since the appearance might be produced by a 

 difference in the refractive power of different jiarts of the cor- 

 pxiscle. 



This objection we think completeiv met; 



1st. By their reflecting the erect image of any opaque body 

 placed between them and the light, precisely as a concave lens 

 would do. 



2dly. By the appearance presented by the particles when 

 viewed dry, as opaque bodies. When illuminated by the whole 

 of the Leiberkuhn, the entire margin is enlightened, and in 

 most of the particles there is besides a broad inner ring of 

 considerable brightness ; whilst the centre, and the space be- 

 tween the two rings, is completely dark. On half the Leiber- 

 kuhn being covered, the rings are reduced to semicircles, the 

 outer one being opposite to the light side, and the inner to the 

 darkened side of the speculum. 



3dly. When fluid blood having been placed between two 

 slips of glass, the particles happen to be at right angles to 

 the surfaces of the glass, so as to be seen in profile, the two 

 concave surfaces are visible at the same time, or alternately, 

 but more distinctly, if the particles slightly vacillate. 



The concavity of the disks is, however, extren)ely trifling ; 

 and under particular circumstances, in a iev; of the particles, 

 the surface is to all appearance quite flat. 



Notwithstanding the great uniformitj- in the size of the par- 

 ticles of the blood, so long as they retain, unimpaired, the form 

 which they possess on escaping from the body, their real mag- 

 nitude has been so variously estimated, that we judged it worth 

 while to attempt a new measurement. In doing so, we adopted 

 a method somewhat different from those hitherto employed. 

 A camera lucida is adapted to the eye-piece of the microscope 

 in such a manner that the distance of the paper being ascer- 

 tained, the object may be drawn on a known scale. Tracings 

 of several of the images being made, they wei'e applied to, 

 and compared with, the images of other particles until their 

 accuracy was established. 



The diameter of the particles obtained in this manner may 

 be pretty correctly stated at --^^-^ of an inch. 



The 



