40 INTRODUCTION. 



they professed to have seen should be extremely discordant. But 

 from the moment that the visual image presented by a well-con- 

 structed Microscope, gave almost as perfect an idea of the object, 

 as we could have obtained from the sight of the object itself, if 

 enlarged to the same size and viewed with the unassisted eye, 

 Microscopic observations admitted of nearly the same certainty as 

 observations of any other class ; it being only in a comparatively 

 small number of cases, that a doubt can fairly remain about any 

 question of fact, as to which the Microscope can be expected to 

 inform us. 1 



Another fallacy, common like the last to all observations, but 

 with which the Microscopic observations of former times were 

 perhaps especially chargeable, arises from a want of due atten- 

 tion to the conditions under which the observations are made. 

 Thus one observer described the Human Blood-corpuscles as 

 flattened disks resembling pieces of money, another as slightly 

 concave on each surface, a third as slightly convex, a fourth as 

 highly convex, and a fifth as globular ; and the former preva- 

 lence of the last opinion, is marked by the habit which still 

 lingers in popular phraseology, of designating these bodies as 

 "blood-globules." Yet all microscopists are now agreed, that 

 their real form, when examined in freshly drawn blood, is that of 

 circular disks, with slightly concave surfaces ; and the diversity 

 in previous statements was simply due to the alteration effected 

 in the shape of these disks, by the action of water or other 

 liquids added for the sake of dilution ; the effect of this being to 

 render their surfaces first flat, then slightly convex, then more 

 highly convex, at last changing their form to that of perfect 

 spheres. But microscopical inquiries are not in themselves more 

 liable to fallacies of this description, than are any other kinds of 

 scientific investigation ; and it will always be found here, as well 

 as elsewhere, that good instruments and competent observers 

 being presupposed the accordance in results will be precisely 

 proportional to the accordance of conditions, that is, to the simi- 

 larity of the objects, the similarity of the treatment to which they 

 may be subjected, and the similarity of the mode in which they 

 may be viewed. 2 



The more completely, therefore, the statements of Microscopic 

 observers are kept free from those fallacies, to which observa- 

 tions of any kind are liable, wherein due care has not been taken 



1 One of the most remarkable of the quastiones vexatce at present agitated, is the 

 nature of the markings on the siliceous valves of Diatomaccce (Chap. VI); some ob- 

 servers affirming those spots of the surface to be elevations, which others consider to 

 be depressions. The difference is here one of interpretation, rather than of direct observa- 

 tion ; the nature of the case preventing that kind of view of the object, which could 

 leave no doubt as to the fact; and the conclusion formed being one of inference from a 

 variety of appearances, which will differently impress the minds of different individuals. 



2 In objects of the most difficult class, such as the Diatomacece, this last point is one of 

 fundamental importance; very different appearances being presented by the same object, 

 according to the mode in which it is illuminated, and the focal adjustment of the object- 

 glass under which it is examined. See Chap. VI. 



