4 AQUATIC MICROSCOPY FOR BEGINNERS. 



and awkward case, one must be selected and sep- 

 arated, for the perverse thing usually comes out of the 

 pocket upside down, and it is of course desirable that the 

 highest-power glass shall be next to and nearest the 

 object, while those not needed are turned to one side, 

 making a series of operations that take time, both 

 hands, and considerable patience if you are anxious to 

 examine the specimen. Your companion will have 

 finished the work with the single glass, and will be 

 telling you how the object looks, before your compli- 

 cated affair is ready to begin, provided you are not 

 wise enough to have avoided the combination pocket- 

 lens. And if the whole number of lenses is used, at 

 once, ,the working-distance is usually so short that 

 the observer's head or hat-brim shuts off most of the 

 light, so that very little of the object can be seen, and 

 that little with difficulty. To see at one view so small 

 a portion as these higher-power combinations always 

 show, and to be compelled to pass the lens over so 

 many small portions before an idea of the whole sur- 

 face can be obtained, is, to say the least, not satis- 

 factory; unless the observer is familiar with the entire 

 object, and with the relation and arrangement of all 

 the parts, a low-power pocket-lens is the most useful, 

 and the one to be commended. 



The reader perceives that this matter of short focus 

 is an important one; indeed the usefulness of the pock- 

 et-lens to a great extent depends upon it. Reject 

 without hesitation the simple lens whose focus is so 

 short that it must be held almost in contact with the 

 object. 



: Some time ago a rather expensive instrument called 

 the "Craig Microscope" was extensively advertised, 



