128 



PEACTICAL TREATISE ON THE MICROSCOPE. 



placed close to the lamp; by means of the jointed arm, 

 the light may be reflected from it upon any object, how- 

 ever large, on the stage. This is much better than a 

 Lieberkuhn for most purposes ; for, with the latter, the ob- 

 jects cannot exceed a certain size, otherwise the greater portion 

 of light from the mirror will be intercepted in its passage ; it 

 has also this advantage over the Lieberkuhn, that not 

 only is a greater amount of light condensed upon any 

 object, but being thrown obliquely, many minute markings 

 can be seen, which the vertically reflected light is imable to 

 bring out. 



Dark Stops or Wells. — These consist of small cup-like 

 pieces of brass, mounted on wire stems or supports ; the shapes 

 ^^^ _^^ generally employed are represented by fig. 79. 

 ^^P p™«il They are used with the Lieberkuhns, and 

 three diflrerent sizes are usually supplied with 

 the best microscopes, the largest being always 

 employed with the lowest power object-glasses. 

 Their use is to cut off all the rays of light that 

 would otherwise pass into the object-glass, 

 hence they are required in all cases where the 

 object to be viewed is transparent. The long 

 stem fits into a small arm attached to the 

 under surface of the stage, and capable of 

 being moved into the centre of the aperture 

 therein, and by it the well at the top can be 

 I raised up so high, as nearly to touch the object 

 itself; the cup-shaped form is used, in order 

 Fig. 79. that the bottom may not be sufiiciently illumi- 

 nated to form a light ground to the object, 

 which might happen if a disc were employed. 



Forceps. — For the purpose of holding minute objects, such 

 as parts of plants, or insects, to be examined either as trans- 

 parent or opaque objects, various forms of forceps have been 

 contrived. The most useful of these is represented by fig. 80. 

 It consists of a piece of steel wire, about three inches long, 

 which slides through a small tube, connected to a stout pin by 

 means of a cradle-joint ; to one end of the wire is attached a 



