18 THE USE OF THE MICROSCOPE 



SO that there is no need to rotate the eye in relation to the 

 lens, as in looking through lenses fixed to the head (specta- 

 cles) ; or even to rotate the head slightly, as in looking 

 through the eyelens of a compound microscope, with a 

 stationary object. 



No doubt the best hand or pocket lenses for magni- 

 fications from 6 to 10 are the usual triplets (Fig. 2 ) which 

 are thick, symmetrical combinations of a biconvex lens, 

 with a diverging meniscus of a more highly refractive and 

 dispersive glass cemented to each side. Somewhat 

 similar, but thinner, triple lenses were made by Steinheil, in 

 1865, and they have been recalculated since. (Triplets 

 corrected only for paraxial rays [Woodworth] are, of course, 

 not aplanatic, in the strict sense of the word.) Triplets 

 with magnifications up to 10, are, in the writer's opinion, 

 optically superior in the images they give to those of 

 higher magnifications. The latter are also difficult to 

 hold steady and focus. If higher magnifications are 

 needed in special cases, they are provided by one optical 

 firm in a series of three small anastigmatic lenses of four 

 components each, magnifying up to 27 times. 



Field of View and Aperture. — ^In the use of an ordinary 

 corrected lens which is larger than the pupil of the observer's 

 eye, the breadth of the field increases with the diameter of 

 the lens (Fig. 4), the eye, of course, being kept close to the 

 lens. The eye should be near the lens, for the width of the 

 field diminishes as the distance of the lens from the eye 

 increases. A large field is useful in searching over a speci- 

 men; and, hence, large lens combinations are usually to 

 be preferred to smaller ones of similar magnification. Also, 

 the lowest magnification which will show clearly the 

 details wanted should be taken, because of the larger portion 

 of the object seen at once. 



The numerical aperture measures the amount of fight 

 received from the object, and is the sine of half the angle of 

 the cone of light which will pass through the pupil into the 

 eye from any point in the object (for a lens in air). The 

 numerical aperture, called here simply aperture, is 



