THE MICROSCOPE AND ITS PARTS. 7 



desirable. The prolonged contraction of the facial mus- 

 cles necessary to keep it in place is very fatiguing, and 

 the vapor always evaporating from the front of the 

 eye being confined within the tube is sure to condense 

 on the lens and obscure the object. Everything a watch- 

 maker's glass will do, a good pocket-lens will accomplish. 



A "Coddington lens" is admirable in many respects. 

 Its magnifying power is great, the image it forms is ex- 

 cellent, the field of view is good, but the focus is usually 

 unpleasantly short. This, aside from its cost, is its only 

 objectionable feature. It is named after the gentleman 

 who first brought it to the notice of the opticians, and 

 not, as it should have been, after Sir David Brewster, 

 its inventor. It consists of a sphere of glass with a deep 

 groove cut around its centre, and filled with a black ce- 

 ment, which acts as a diaphragm to cut off certain rays 

 of light whose presence and action would be undesira- 

 ble, as they would interfere with the formation of a clear 

 and sharply outlined image. 



The reader may be surprised to learn that there are 

 people who do not know how to focus a lens. I have 

 seen such persons take the glass as if they were afraid 

 of it. They extend it towards the object in a hesitating 

 way, move it about irregularly for a few moments, throw 

 back the head, look cross-eyed, and say, " Oh yes ; I see. 

 How beautiful! And how very queer it looks!" I 

 once offered a lady an opera-glass, which she put to her 

 eyes and never touched the adjustment wheel that alters 

 the length of the tubes and focuses the lenses on the 



