20 THE USE OF THE MICTiOSCOPE 



Mode of Using Lens. -In all cases, the hand lens, lor 

 the best results in sharpness of image or extent of field, 

 must be held as close to the eye as spectacle glasses, and at 

 right angles to the axis of vision; the flat or the concave 

 side being next to the eye. Any slight obliquity may 

 perceptibly injure the definition of planoconvex corrected 

 lenses. This is not so much the case, however, with the 

 thick triplet; in which, because of its approximation to 

 the spherical form, a slight obliquit}", which is almost 

 inevitable in the ordinary use of a hand lens, may not be 

 markedly prejudicial. The triplet can also be used with 

 either side next the eye, as it is symmetrical in this respect. 



It is advantageous to shade the eye which is looking 

 through a lens from extraneous light. Many years ago, 

 Leeuwenhoek mounted each of his little lenses in the center 

 of a sheet of metal, and this doubtless helped him in getting 

 his remarkable results. Hence the margin of the holder 

 around a hand lens may well be fairly wide and blackened. 



Besides their use in examining objects in the field, 

 hand magnifiers of 3 to 10 times magnification are essential 

 for examining the back and front lens surfaces of objectives, 

 the upper surface of the eyelens of the eyepiece, etc., to 

 ensure optical cleanliness, and also to note the size of the 

 condenser circle at the eyepoint. 



In using hand lenses with magnifications of 3 to 10, 

 if the optical rules are disregarded, one takes an uncorrected 

 lens, and holds it at reading distance from the eye, with 

 the other eye uncovered. When the rules are regarded, 

 a corrected doublet or triplet is fixed in a holder with 

 ground glass over the unused eye, so that the lens is kept 

 at right angles to the optic axis, and 12 millimeters from the 

 cornea, or close to spectacle glasses. One looks through the 

 center of the lens, and moves it over the object, following 

 it mainly by movements of the head; instead of, as in 

 spectacle glasses and verants, rotating the eye also. 



Rules Disregarded Rules Regarded 



1. Small field. Largest possible field. 



2. Details blurred. Sharpest details. 



