THE MICROSCOPE. 83 



"doublets" (two pieces of glass), "triplets'" (three pieces), 

 "platyscopic lenses," etc. A section of a triplet is shown 

 in fig. 32. A good triplet gives a perfectly flat field and is 

 free from rainbow hues around the object viewed. 



Some means of support should be devised to hold the 

 simple lens. A very simple one may be made by means of 

 a block of wood, two bits of stiff iron wire, and a couple of 

 corks. The block of wood should be used as a base. In 

 its centre one of the pieces of wire should be fixed in an up- 

 right position. On this wire one of the corks should be 



FIG. 33- 



made to slide up and down freely, but not so loosely as to 

 slip. Through this cork the second rod should move with 

 the same freedom as the first, but at right angles ; and at its 

 farther extremity, the second cork, which is shaped to hold 

 the lens, should be fixed. The corks used in this way afford 

 a very smooth motion and the apparatus produces good 

 results. 



A rather more expensive piece of apparatus for this pur- 

 pose is that shown in fig. 33, and which hardly needs any 

 explanation. The whole is made of brass and is especially 

 adapted for the usual style of mounting triplets. The two 



