THE MICROSCOPE 135 



bulbs may be frosted by dipping in a 15 per cent, solution 

 of caustic soda and allowing to dry. 



Two eyepieces are sufficient, and the lower-power ones 

 are to be preferred, such as the B and c of the English, 

 or the 2 and 3 of the Continental, makers. Although 

 increased magnification can be obtained by the use of a 

 high-power eyepiece, it is at the expense of definition, 

 the image losing its sharpness, because the eyepiece mag- 

 nifies the image formed by the objective, and any imper- 

 fections in the latter are made more apparent, so that the 

 use of very high eyepieces is not to be recommended, 

 except with the finest lenses ; moreover, as will be pointed 

 out later, it is useless to increase the amplification beyond 

 a certain point. 



With regard to the length of the tube of the microscope, 

 this differs in the English and Continental systems. The 

 standard English tube-length is 8-75 in., the Continental 

 is 6-3 in., and is usually adopted, but the longer tube gives 

 greater amplification. The tube of the microscope is 

 generally provided with an inner, or draw-tube, by means 

 of which its length can be nearly doubled ; this gives 

 increased amplification, but at the expense of definition, 

 at least with the higher powers which are corrected or 

 adjusted for a definite tube-length. 



The lenses or objectives must next be considered. 



For powers higher than the J-in., or thereabouts, it is 

 advisable, for many reasons, to employ the immersion 

 system of objectives. With these lenses a drop either of 

 water, in the water-immersion system, or of cedar oil, in 

 the oil- immersion one, is placed on the cover- glass, and 

 the objective is racked down so that its front lens touches 

 and is immersed in either the water or oil, as the case 

 may be. It is a good plan then to raise the objective very 

 slightly by means of the coarse adjustment, still, however, 

 keeping it in contact with the drop of water or oil. The 



