50 



Which Eye to Use. In a binocular instrument 

 both eyes are used, but in a monocular only one is 

 used, and it depends upon a trial which is best suited. 

 A large proportion of persons are afflicted with astig- 

 matism, often without knowing it, and when this 

 exists it may be in one eye or when in both, may be 

 to a greater extent in one than in the other. Its 

 presence may prevent the eye from observing fine 

 detail ; but whichever eye is found to be best suited 

 should be used. When both eyes are alike it is some- 

 times advisable to change from one to the other. 



It should be made a habit at the outset and strictly 

 adhered to, to keep both eyes open.- A little difficulty 

 may be found to do this, as the eye which is free will 

 probably observe the objects upon the table ; but as 

 soon as the mind becomes fixed upon what it sees in 

 the microscope, this impression disappears. After a 

 time it will be found to require no exertion and will 

 certainly add to the ease and comfort of the manip- 

 ulator while working. 



The Ward Eye Shade, Fig. 15, will prove of as- 

 sistance in acquir- 

 ing the above men- 

 tioned habit, and 

 ebsides this, effect- 

 ually excludes the 



Fig. 15. light from the eyes. 



It is made of hard rubber and is attached to the tube 

 of the microscope. 



