164 ERRORS OF INTERPRETATION. 



stopped out by the interposition of a ' dark well ' or of a black 

 disk, of such a size as to fill the field given by the particular 

 Objective employed, but not to pass much beyond it. Opaque 

 objects that are permanently mounted either upon cardboard disks 

 or in the slides specially provided for them, may be presented to 

 the Microscope in a considerable variety of directions by means 

 of Morris's Object-holder (Fig. 77), which, however, can only be 

 employed with side-illumination. If it be desired to make the 

 most advantageous use of this instrument, objects mounted in 

 slides should be so placed that the parts to be brought into view 

 by its tilting movement may look towards the long edges of the 

 slide ; since it is obvious that a much greater inclination may be 

 given to it in either of these directions, than in the direction of 

 either of its extremities. 



125. Errors of Interpretation. — The correctness of the con- 

 clusions which the Microscopist will draw regarding the nature of 

 any object, from the visual appearances which it presents to him 

 when examined in the various modes now specified, will necessarily 

 depend in a great degree upon his previous experience in Microscopic 

 observation, and upon his knowledge of the class of bodies to which 

 the particular specimen may belong. Not only are observations of 

 any kind liable to certain fallacies, arising out of the previous 

 notions which the observer may entertain in regard to the consti- 

 tution of the objects or the nature of the actions to which his 

 attention is directed, but even the most practised observer is apt to 

 take no note of such phenomena as his mind is not prepared to 

 appreciate. Thus, for example, it cannot be doubted that many 

 Physiologists must have seen those appearances in thin slices of 

 Cartilage which are now interpreted as denoting its cellular orga- 

 nization, without in the least degree suspecting their real import, 

 which Schwann was the first to deduce from the study of the 

 development of that tissue. It was not known before his time 

 "what cells mean" in animal organization; and the visual 

 appearances, which now suggest the idea of them to the mind of 

 even the tyro in the study of Histology, passed almost entirely un- 

 noticed by keen- sighted and intelligent Microscopists previously to 

 1839. Errors and imperfections of this kind can only be cor- 

 rected, it is obvious, by general advance in scientific knowledge ; 

 but the history of them affords a useful warning against hasty con- 

 clusions drawn from a too cursory examination. If the history of 

 almost any scientific investigation were fully made known, it 

 would generally appear that the stability and completeness of the 

 conclusions finally arrived-at had only been attained after many 

 modifications, or even entire alterations, of doctrine. And it is, 

 therefore, of such great importance to the correctness of our conclu- 

 sions as to be almost essential, that they should not be finally formed 

 and announced until they have been tested in every conceivable mode. 

 It is due to Science that it should be burdened with as few false 



