GLASGOW SOCIETY OF FIELD NATURALISTS. 135 



costs about six pounds. A microscope lamp must have its burner 

 adjustable in vertical dii'ections and allow of its being revolved, 

 or it does not deserve the name, although various patterns not 

 having these properties are made and sold as microscope lamps. 

 For bringing out markings and for obtaining very intense illumin- 

 ation the narrow side of the wick should be used : indeed, various 

 sides of the flame should always be tried, as by this means the 

 best intensity of light can easily be ascertained. 



In the illumination of objects the rays ought to be parallel in 

 nearly all cases. By paying attention to this, most structures of 

 transparent objects can be made out by the mirror or rectangular 

 prism alone. The angle of illumination should not be excessive, 

 and should not exceed 90 degs. Objects are either opaque or 

 transparent, and the latter may be viewed by transmitted or by 

 reflected light. 



The simplest means of illuminating opaque objects is by means of 

 the so-called bull's-eye condenser, which is a plano-convex lens 

 mounted on a separate stand, or fitting into the stage or the stand 

 of the mici'oscope ; the latter form is called a stage condenser, and 

 is not very useful. The convex side should always be turned towards 

 the source of light in order to obtain parallel rays, else by the 

 crossing and recrossing of an infinite number of rays all the 

 shadows will be destroyed which are absolutely requisite to bring 

 out the shape and nature of the surface of the object under 

 examination. Greater brilliancy is obtained by passing the rays 

 first through a bull's-eye condenser on to a silvered side reflector of 

 parabolic surface, often called a specukim, which in its turn 

 reflects the rays on the object. Objects which are mounted on an 

 opaque ground, about |-inch or less diameter, may be illuminated 

 by means of the LieherkuJin (figure 13), which resembles the 

 mouth-piece of a wind instrument, and is silvered on its hollow 

 parabolic surface. It is slid over the object-glass, and the rays are 

 reflected on it from the plane mirror. The Lieberkuhn's parabolic 

 surface reflects in its turn the light on the object, which is placed 

 in its focus. A separate Liebei-kuhn is required for each object- 

 glass, and the highest power under which an opaque object can be 

 illuminated with a Lieberkuhn, a parabolic side reflector, or 

 the bull's-eye condenser, is the ^-inch, or a very narrow-angled 

 |-inch glass, provided they have a long working distance. Many 

 microscopists condemn the Lieberkuhn as thi-owing light from all 



