But in order to determine whether a defect exists, of 

 which the possessor is not aware, observe closely first 

 with one eye then with the other an object, preferably 

 one with fine striations such as a diatom, to learn 

 whether with one eye a better view is obtained than 

 with the other and use the one giving the best results. 



Make it a habit at the outset to keep both eyes 

 open. 



There is one point over the lens called the eye-point, 

 Fig. 39, e p, at which the rays cross within the smallest 

 compass, and this is the proper position for the eye as 

 the largest number of rays enter it. When above or 

 below this point the size of field will be reduced or 

 shadows or colors will appear in it. In low power 

 eyepieces the eye-point is some distance from the lens ; 

 in high powers quite close to it in fact in some so 

 close that the eyelashes rest upon the lens and some- 

 times appear in the field as dark bars. Generally 

 speaking the best point is where the entire field is seen 

 and its margin (diaphragm) sharply defined. 



What Objects to Use. Suitable objects for 

 preliminary work in leading the student to a skillful 

 use of the instrument and to give him proper judgment 

 in testing the capacity of objectives are also valuable. 



Low Powers Proboscis of blow-fly. This should 

 be flat and transparent. For i inch, 2-3 and 1-2 inch 

 objectives the scales from Lepisma saccharina. 



132 



