180 MANAGEMENT OF THE MICROSCOPE. 



berkiihn ; thus enabling it to afford a kind of illumination, which, 

 as already remarked, is usually much more valuable than that 

 produced by the nearly perpendicular rays sent down by it on 

 the object, when the mirror is placed in the axis. Whenever the 

 Lieberkiihn is employed, care must be taken that the direct light 

 from the mirror be entirely 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. An ingenious combination of a hemispherical Lie- 

 berkiihn with the Paraboloid ( 61) has been devised by Mr. 

 Wenham, for the illumination of minute opaque objects by very 

 oblique rays, 1 and Mr. C. Brooke has attached a small plane spe- 

 culum to objectives of l-8th and l-12th inch focus (which cannot 

 be otherwise advantageously employed with that illuminator), in 

 such a manner that its surface is level with, or very little below, 

 that of the outer lens, so as to reflect downwards upon the object 

 those extreme pencils of rays which pass by the aperture of the 

 object-glass. In either case, an oblique illumination from one 

 side only may be obtained, by shutting off either half of the lower 

 aperture of the paraboloid. These contrivances for the examina- 

 tion of minute objects with high powers by incident light,- have 

 scarcely yet received the attention they deserve. 



95. ^Errors of Interpretation. The correctness of the conclu- 

 sions 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 neces- 

 sarily depend in 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, as already remarked (Intro- 

 duction, pp. 39-41), to certain fallacies arising out of the previous 

 notions which the observer may entertain, in regard to the con- 

 stitution 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 pre- 

 pared 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 organization, 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 retinal pictures which now suggest the idea of them to the 

 mind of even the tyro in the study of Histology (p. 54), passed 

 almost entirely unnoticed by keen-sighted and intelligent Micro- 

 scopists previously to 1839. Errors and imperfections of this 

 kind can only be corrected, it is obvious, by general advance in 

 scientific knowledge ; but the history of them affords a useful 



1 "Quart. Journ. of Microsc. Science," vol. ii, p. 155. 



