APPENDIX 515 



Uncontrolled refraction of light rays, producing distortion of 

 image. 



1 . Dip the front of a simple lens in water, and, without 

 wiping it, look through it in the letters of a printed page. 

 You have altered the curvature of the surface first met by 

 the rays of light coming from the print, deflecting them out 

 of their proper course. 



2. With the low power objective in place on the micro- 

 scope and clean, the eyepiece in place, and some object 

 clearly in focus, touch the front of the objective lightly with 

 the moist finger, and observe how the appearance of the 

 object is altered. The fingers are never optically clean. 

 The slightest deposit on the surface of a lens disturbs its 

 refracting harmony. Do not touch the glass of your lenses 

 with the finger again, nor with anything else, except on the 

 occasions (which will then be rare) when it necessary to 

 clean them. 



3. To clean a lens, moisten it with the breath and wipe 

 dry with soft lens paper (or with a very soft old linen hand- 

 kerchief) ; when more than this is needed, take it to the 

 instructor. 



4. Place a little tuft of green algae upon a slide, wet, and 

 examine it with a lens. Then lower a cover glass upon it, 

 fill up beneath the cover with water and examine again. 

 The cover gives the flat upper surface that is necessary to 

 prevent uncontrolled refraction and distortion of image. 

 Complete immersion in a watchglass of water would give a 

 similar result. You cannot well examine an object half 

 wet and half dry. Dry objects may be examined with low 

 power lenses uncovered, but you must always use a cover 

 glass with the high power objective. 



5. Air entangled under the cover glass is a frequent 

 source of trouble. The air may cling to the cover will be 

 likely to do so if the cover be dropped flat upon the object. 



