USE OF TIIK HAM) MAdXIFIER 15 



least equally important, for the errors due to the spherical 

 shape of the surfaces. Thus, though such lenses are simply 

 called achromatic, the spherical aberrations are also much 

 lessened, especially when the lens is held in its calculated 

 position, close to the eye. Some large lenses, in reading 

 glasses and microscope condensers, are now corrected for 

 spherical aberration by grinding them to empirical aspheric 

 curves (making them aplanatic) ; but this method has 

 apparently not yet been so successful with small lenses. 



In most cases, corrected lenses can be identified by notic- 

 ing the junction of the cemented components at the edge 

 of the lens; or by observing the feeble image of a light 

 reflected from the cemented surface, as compared w'ith the 

 bright images reflected from the air-glass and glass-air 

 surfaces. 



An uncorrected magnifying lens is made from a single 

 piece of glass, or from two lenses of the same glass spaced 

 apart; and its use, whether as a simple magnifier, Codding- 

 ton lens, or uncorrected doublet, is nearly a hundred years 

 out of date. The image given by such a lens has nearly all 

 possible faults, as can be seen by comparing its performance 

 on a page of fine print with that of a corrected doublet or 

 triplet of the same magnification. 



Compensating the Unoccupied Eye. — In these single 

 magnifiers, one eye is of course unoccupied. The following 

 is quoted from a note by the waiter on "Compensating the 

 Unoccupied Eye in Monocular Instruments" (35). 



When using a hand lens, an ordinary single-tube microscope, or any 

 other optical instrument made for one eye, three points at least may be 

 considered with regard to balancing the two eyes: (1) the intensity and 

 angle of the light passing through the two pupils may be made roughly 

 equal, so that the two irises may not tend to be in conflict with regard 

 to contraction or expansion; (2) an arrangement may be made to facili- 

 tate the axes of the two eyes converging to the same point, and this 

 point is best, in many or most cases, if situated at an indefinite distance 

 [that is, so far off that the axes of the two eyes are parallel]; (3) the 

 accommodation of the two eyes, which is more or less linked with their 

 convergence, may be kept api)roximately the same. 



