158 PRACTICAL TBEATISE ON THE MICEOSCOPE. 



To Clean Chimneys of Lamps. — In all cases where a chim- 

 ney is used with a lamp, it is liable at times to become dull 

 and smoky ; this cannot be washed off by water alone, a small 

 quantity of hydrochloric acid added to the water will at once 

 remove it. When this is not at hand, a mixture of common 

 salt and yinegar will answer the same purpose. A piece of 

 flannel fastened to the end of a stick and dipped into either of 

 these liquids will serve to convey it to all parts of the glass. 

 Care must be taken to have the chimney wiped perfectly dry 

 before it is put over the flame, otherwise it is liable to crack, 

 and when the lamp is first lighted, it is advisable to have the 

 entire circle of the vidck lighted before the chimney is put on ; 

 some persons put the chimney on when only a portion of the 

 wick is lighted, this is always attended with risk. 



CHAPTER V. 



ON THE MAGNIPTING POWERS USED WITH SIMPLE AND 

 ACHROMATIC COMPOUND MICROSCOPES. 



It has been previously stated, at page 64, that the magnifying 

 powers employed with the simple microscopes may be divided 

 into those consisting of one lens only, or into those of two or 

 three lenses combined, and termed, in consequence, either 

 doublets or triplets. The former was stated to answer ex- 

 ceedingly well for all the lowest powers, and the latter for 

 the highest. It would be foreign to our purpose to enter in 

 detail into all the different methods of constructing the mag- 

 nifying powers or lenses that from time to time have occupied 

 the attention of the learned in this and other countries, nor 

 will it be necessary to trace the alteration in course that the 

 rays of light undergo in their passage through lenses of the 

 various figures employed in optical instruments, as these will 

 be found fully described in the works exclusively devoted to 

 this subject ; but, in order to understand in what an achro- 

 matic object-glass differs from one of the ordinary construction, 



