37 



Keep the eye comfortably near to the upper 

 surface of the lens, as the angular view is in- 

 creased and there is the least spherical aberration 

 and the focal distance is the greatest. This can 

 be easily tested by gradually increasing the dis- 

 tance between the eye and lens, when it will be 

 found that the lens must be brought nearer to the 

 object. In single lenses the spherical and chro- 

 matic aberrations become more pronounced and 

 the field smaller. 



As magnifiers which are used on opaque objects 

 -those which are not transparent and which are 

 illuminated by reflected light not transmitted 

 through the object- -a position should be chosen 

 toward a window or flame, which will allow the 

 greatest amount of light to reach the object. If a 

 hat is worn, place this back on the head so that 

 the rim or shield will not cut off the light. 



In holding the object in one hand, take the 

 magnifier between the thumb and forefinger of 

 the other and place the fingers of hand holding 

 the lens in such a manner that they shall rest 

 upon the other hand ; this will insure steadiness 

 between the lens and object and will add consider- 

 ably to the comfort of working. 



While it seldom occurs that magnifiers are made 

 with other than double convex surfaces, single 

 achromatic lenses are sometimes used which are 

 plane convex. In these the plane side should 



