USE OF THE HAND MAGNIFIER 25 



Practical Points 



1. A corrected hand lens should Ix' used in scientific work. 



2. The plane or the concave side of an unsymmetrical corrected 

 lens should be next the eye. 



3. The hand lens should be held as close to the eye as are 

 spectacle glasses; that is, about 12 millimeters. 



4. In using a hand lens, the unoccupied eye may be compen- 

 sated by a disc of ground glass in a frame. 



5. The binocular is to be preferred to the single lens. 



6. The spectacles worn when using a well-corrected hand lens 

 should (usually) be distance spectacles. 



7. Glasses correcting astigmatism or focal differences should 

 be retained. 



8. The surface of the spectacle glasses should be kept at right 

 angles to the optic axis, by duly inclining the instrument. 



9. With the binocular magnifiers, both the distance between 

 eyepoints and the focal distance should be adjusted between 

 the possible extremes. The focal distance should be near the 

 maximum. 



10. If reading spectacles are worn when using a hand lens, it 

 will be found that the lens gives the best image when held about 

 3 millimeters towards the nasal side of the eye. 



11. Since reading spectacles may have about 5 millimeters 

 less distance between centers than distance spectacles, reading 

 glasses should be worn with binoculars with converging tubes, 

 unless these are corrected for distance vision. In the latter case 

 (Zeiss, Leitz, etc.), distance glasses should be fitted as eyepiece 

 caps on the instrument. Otherwise, eyestrain results from bad 

 centering. 



