xxxvi INTRODUCTION. 



structure of the outer surface is in general easily determined by examining it with reflected 

 light, i. e. as an opaque object illuminated by the Lieberkiilm or side-condenser ; but when 

 the objects are small, sufficient light cannot be thrown upon them with ordinary conden>'i - : 

 recourse must then be had to the opaque reflectors mentioned at p. xxi. 



b. The appearances presented must also be controlled by those resulting from the action 

 of transmitted light. And here we meet with a difficult task, in accomplishing which, the 

 f olloAving questions are constantly presenting themselves : Do certain spots, lines, or other 

 markings visible upon the surface represent elevations or depressions ? Are they cavities 

 in the outer portion or layer of the object ? Are they foramina or holes ? Are they 

 granules of pigment, or rows of them ? Do the lines represent a true lined structure, or 

 are they optical illusions ? Is the surface smooth and free from markings ? The methods 

 of answering these questions must vary so greatly, according to the nature of the object, 

 its size, &c., that it would be almost impossible to lay them down by rule. The following 

 considerations, however, are of most importance. 



c. In many cases where structural appearances are visible at the surface of an object, 

 their true situation above or beneath the surface may be determined by raising the object- 

 glass above the focus of the surface. On then carefully and gradually depressing the 

 object-glass with the fine movement, the structure first brought into focus is the uppermost. 

 Thus, the inner surface of the under menibrame of the elytrumof the stag-beetle (Luctuiu* 

 cervus) is covered with very minute hairs projecting from the surface (PI. 34. fig. 2). On 

 placing this with the inner side uppermost and adjusting the object-glass as just 

 described, the hairs are distinctly brought into focus before the surface of the membrane. 

 Hence they are situated upon the surface ; whereas, had the surface of the membrane 

 been brought into view before the hairs, it must have been concluded that the latter 

 were situated on a plane below this. It may be stated that the surface of a membrane is 

 recognized to be in focus by certain irregular granules, molecules, or wrinkles mostly 

 visible upon it. 



d. Frequently, when hairs, filaments, ov spines project from a surface, their relative 

 position may be determined by examining the margin of the object if it be rounded, or 

 the margin of a fold if it be flat and membranous as in the case of ciliated bodies, 

 Infusoria, &c. 



e. Cilia upon the surface of an object are sometimes so minute and transparent as to be 

 with difficulty detected ; they can however always be made evident, when present, by the 

 following means : 1. Drying the object j they then become much darker from refraction. 

 2. Dyeing the object with solution of iodine; drying the object after the addition of the 

 latter solution is sometimes advantageous. 3. Mixing insoluble coloured particles, as those 

 of lampblack, or Prussian blue, with the water in which the objects are contained; of 

 course this is only of use if the objects be living; the particles will then be set in motion, 

 and their motion may be distinguished from molecular motion by the definite direction in 

 which the particles move. 



/. The nature of many markings, spots, &c. in transparent objects is best determined 

 by Dujardin's method, viz. that of comparing at different foci the effects of the refraction 

 of the transmitted light produced by the markings themselves, and the substances in which 

 they are situated ; and these phenomena may be conveniently illustrated by their occurrence 

 in known objects. If a drop of oil of turpentine, which has been digested with alkanet 

 root so as to become coloured, be placed upon a slide, a drop of water added to it, a thin 



