186 USJi'or THE MICROSCOPE. 



CHAPTER II. 



ON THE ILLUMINATION OF OBJECTS. 



All the objects required to be examined by the microscope 

 belong to one of two classes, either the transparent or the 

 opaque. The methods of illumination differ for each class: in 

 the former the light is generally reflected upon them by means 

 of a plane or convex mirror, whilst in the latter it is either 

 condensed upon them by a lens or else by some modification 

 of the silvered cup, termed, from its inventor, the Lieberkuhn. 

 As the perfect illumination of objects is of the utmost import- 

 ance, nay, almost as essential as a good glass to examine them 

 with, we win enter somewhat in detail into the various plans 

 to be adopted, in order that all the characters of the subject 

 under investigation may be brought out and rendered per- 

 fectly distinct. In order to effect this, it is requisite that it 

 be viewed alternately under every description of light, 

 whether strong or faint, oblique or direct ; or whether the 

 light be reflected through it from a mirror, as in the case of a 

 transparent object, or be condensed upon it by a lens as 

 for an opaque ; in short, every new subject should be 

 viewed under all the various conditions subsequently to be 

 enumerated. 



Transparent Objects. — When a great amount of light is 

 required, the plan generally adopted is to use the concave 

 mirror, which should be so contrived as to slide up and down 

 upon a stem, so that it may be brought near the stage or be 

 slid away from it, and, by means of a universal joint, be made 

 capable of being turned in every direction ; when rays of 

 light from a lamp or candle placed beyond its principal focus, 

 fall upon a concave mirror, they are rendered convergent, and 

 the distance of the mirror from the stage should, therefore, be 

 so regulated that, if required, the whole of the light reflected 

 from the mirror may be brought to a focus upon the object, 

 or if a weaker light be necessary, then by sliding the mirror 

 either nearer the stage or farther away from it, the desired 



